The PQexec function is adequate for submitting commands in normal, synchronous applications. It has a few deficiencies, however, that can be of importance to some users:

• PQexec waits for the command to be completed. The application might have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response.

• Since the execution of the client application is suspended while it waits for the result, it is hard for the application to decide that it would like to try to cancel the ongoing command. (It can be done from a signal handler, but not otherwise.)

• PQexec can return only one PGresult structure. If the submitted command string contains multiple SQL commands, all but the last PGresult are discarded by PQexec.

• PQexec always collects the command's entire result, buffering it in a single PGresult. While this simplifies error-handling logic for the application, it can be impractical for results containing many rows.

Applications that do not like these limitations can instead use the underlying functions that PQexec is built from: PQsendQuery and PQgetResult. There are also PQsendQueryParams, PQsendPrepare, PQsendQueryPrepared, PQsendDescribePrepared, and PQsendDescribePortal, which can be used with PQgetResult to duplicate the functionality of PQexecParams, PQprepare, PQexecPrepared, PQdescribePrepared, and PQdescribePortal respectively.PQsendQuery

Submits a command to the server without waiting for the result(s). 1 is returned if the command was successfully dispatched and 0 if not (in which case, use PQerrorMessage to get more information about the failure).

int PQsendQuery(PGconn *conn, const char *command);

After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the results. PQsendQuery cannot be called again (on the same connection) until PQgetResult has returned a null pointer, indicating that the command is done.PQsendQueryParams

Submits a command and separate parameters to the server without waiting for the result(s).

int PQsendQueryParams(PGconn *conn,
                      const char *command,
                      int nParams,
                      const Oid *paramTypes,
                      const char * const *paramValues,
                      const int *paramLengths,
                      const int *paramFormats,
                      int resultFormat);

This is equivalent to PQsendQuery except that query parameters can be specified separately from the query string. The function's parameters are handled identically toPQexecParams. Like PQexecParams, it will not work on 2.0-protocol connections, and it allows only one command in the query string.PQsendPrepare

Sends a request to create a prepared statement with the given parameters, without waiting for completion.

int PQsendPrepare(PGconn *conn,
                  const char *stmtName,
                  const char *query,
                  int nParams,
                  const Oid *paramTypes);

This is an asynchronous version of PQprepare: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, call PQgetResult to determine whether the server successfully created the prepared statement. The function's parameters are handled identically to PQprepare. Like PQprepare, it will not work on 2.0-protocol connections.PQsendQueryPrepared

Sends a request to execute a prepared statement with given parameters, without waiting for the result(s).

int PQsendQueryPrepared(PGconn *conn,
                        const char *stmtName,
                        int nParams,
                        const char * const *paramValues,
                        const int *paramLengths,
                        const int *paramFormats,
                        int resultFormat);

This is similar to PQsendQueryParams, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. The function's parameters are handled identically to PQexecPrepared. Like PQexecPrepared, it will not work on 2.0-protocol connections.PQsendDescribePrepared

Submits a request to obtain information about the specified prepared statement, without waiting for completion.

int PQsendDescribePrepared(PGconn *conn, const char *stmtName);

This is an asynchronous version of PQdescribePrepared: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, call PQgetResult to obtain the results. The function's parameters are handled identically to PQdescribePrepared. Like PQdescribePrepared, it will not work on 2.0-protocol connections.PQsendDescribePortal

Submits a request to obtain information about the specified portal, without waiting for completion.

int PQsendDescribePortal(PGconn *conn, const char *portalName);

This is an asynchronous version of PQdescribePortal: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, call PQgetResult to obtain the results. The function's parameters are handled identically to PQdescribePortal. Like PQdescribePortal, it will not work on 2.0-protocol connections.PQgetResult

Waits for the next result from a prior PQsendQuery, PQsendQueryParams, PQsendPrepare, PQsendQueryPrepared, PQsendDescribePrepared, or PQsendDescribePortal call, and returns it. A null pointer is returned when the command is complete and there will be no more results.

PGresult *PQgetResult(PGconn *conn);

PQgetResult must be called repeatedly until it returns a null pointer, indicating that the command is done. (If called when no command is active, PQgetResult will just return a null pointer at once.) Each non-null result from PQgetResult should be processed using the same PGresult accessor functions previously described. Don't forget to free each result object with PQclear when done with it. Note that PQgetResult will block only if a command is active and the necessary response data has not yet been read by PQconsumeInput.

Note

Even when PQresultStatus indicates a fatal error, PQgetResult should be called until it returns a null pointer, to allow libpq to process the error information completely.

Using PQsendQuery and PQgetResult solves one of PQexec's problems: If a command string contains multiple SQL commands, the results of those commands can be obtained individually. (This allows a simple form of overlapped processing, by the way: the client can be handling the results of one command while the server is still working on later queries in the same command string.)

Another frequently-desired feature that can be obtained with PQsendQuery and PQgetResult is retrieving large query results a row at a time. This is discussed in Section 33.5.

By itself, calling PQgetResult will still cause the client to block until the server completes the next SQL command. This can be avoided by proper use of two more functions:PQconsumeInput

If input is available from the server, consume it.

int PQconsumeInput(PGconn *conn);

PQconsumeInput normally returns 1 indicating “no error”, but returns 0 if there was some kind of trouble (in which case PQerrorMessage can be consulted). Note that the result does not say whether any input data was actually collected. After calling PQconsumeInput, the application can check PQisBusy and/or PQnotifies to see if their state has changed.

PQconsumeInput can be called even if the application is not prepared to deal with a result or notification just yet. The function will read available data and save it in a buffer, thereby causing a select() read-ready indication to go away. The application can thus use PQconsumeInput to clear the select() condition immediately, and then examine the results at leisure.PQisBusy

Returns 1 if a command is busy, that is, PQgetResult would block waiting for input. A 0 return indicates that PQgetResult can be called with assurance of not blocking.

int PQisBusy(PGconn *conn);

PQisBusy will not itself attempt to read data from the server; therefore PQconsumeInput must be invoked first, or the busy state will never end.

A typical application using these functions will have a main loop that uses select() or poll() to wait for all the conditions that it must respond to. One of the conditions will be input available from the server, which in terms of select() means readable data on the file descriptor identified by PQsocket. When the main loop detects input ready, it should call PQconsumeInput to read the input. It can then call PQisBusy, followed by PQgetResult if PQisBusy returns false (0). It can also call PQnotifies to detect NOTIFY messages (see Section 33.8).

A client that uses PQsendQuery/PQgetResult can also attempt to cancel a command that is still being processed by the server; see Section 33.6. But regardless of the return value of PQcancel, the application must continue with the normal result-reading sequence using PQgetResult. A successful cancellation will simply cause the command to terminate sooner than it would have otherwise.

By using the functions described above, it is possible to avoid blocking while waiting for input from the database server. However, it is still possible that the application will block waiting to send output to the server. This is relatively uncommon but can happen if very long SQL commands or data values are sent. (It is much more probable if the application sends data via COPY IN, however.) To prevent this possibility and achieve completely nonblocking database operation, the following additional functions can be used.PQsetnonblocking

Sets the nonblocking status of the connection.

int PQsetnonblocking(PGconn *conn, int arg);

Sets the state of the connection to nonblocking if arg is 1, or blocking if arg is 0. Returns 0 if OK, -1 if error.

In the nonblocking state, calls to PQsendQuery, PQputline, PQputnbytes, PQputCopyData, and PQendcopy will not block but instead return an error if they need to be called again.

Note that PQexec does not honor nonblocking mode; if it is called, it will act in blocking fashion anyway.PQisnonblocking

Returns the blocking status of the database connection.

int PQisnonblocking(const PGconn *conn);

Returns 1 if the connection is set to nonblocking mode and 0 if blocking.PQflush

Attempts to flush any queued output data to the server. Returns 0 if successful (or if the send queue is empty), -1 if it failed for some reason, or 1 if it was unable to send all the data in the send queue yet (this case can only occur if the connection is nonblocking).

int PQflush(PGconn *conn);

After sending any command or data on a nonblocking connection, call PQflush. If it returns 1, wait for the socket to become read- or write-ready. If it becomes write-ready, call PQflushagain. If it becomes read-ready, call PQconsumeInput, then call PQflush again. Repeat until PQflush returns 0. (It is necessary to check for read-ready and drain the input with PQconsumeInput, because the server can block trying to send us data, e.g. NOTICE messages, and won't read our data until we read its.) Once PQflush returns 0, wait for the socket to be read-ready and then read the response as described above.

Ordinarily, libpq collects a SQL command's entire result and returns it to the application as a single PGresult. This can be unworkable for commands that return a large number of rows. For such cases, applications can use PQsendQuery and PQgetResult in single-row mode. In this mode, the result row(s) are returned to the application one at a time, as they are received from the server.

To enter single-row mode, call PQsetSingleRowMode immediately after a successful call of PQsendQuery (or a sibling function). This mode selection is effective only for the currently executing query. Then call PQgetResult repeatedly, until it returns null, as documented in Section 33.4. If the query returns any rows, they are returned as individual PGresult objects, which look like normal query results except for having status code PGRES_SINGLE_TUPLE instead of PGRES_TUPLES_OK. After the last row, or immediately if the query returns zero rows, a zero-row object with status PGRES_TUPLES_OK is returned; this is the signal that no more rows will arrive. (But note that it is still necessary to continue calling PQgetResult until it returns null.) All of these PGresult objects will contain the same row description data (column names, types, etc) that an ordinary PGresult object for the query would have. Each object should be freed with PQclear as usual.PQsetSingleRowMode

Select single-row mode for the currently-executing query.

int PQsetSingleRowMode(PGconn *conn);

This function can only be called immediately after PQsendQuery or one of its sibling functions, before any other operation on the connection such as PQconsumeInput or PQgetResult. If called at the correct time, the function activates single-row mode for the current query and returns 1. Otherwise the mode stays unchanged and the function returns 0. In any case, the mode reverts to normal after completion of the current query.

Caution

While processing a query, the server may return some rows and then encounter an error, causing the query to be aborted. Ordinarily, libpq discards any such rows and reports only the error. But in single-row mode, those rows will have already been returned to the application. Hence, the application will see some PGRES_SINGLE_TUPLE PGresult objects followed by a PGRES_FATAL_ERROR object. For proper transactional behavior, the application must be designed to discard or undo whatever has been done with the previously-processed rows, if the query ultimately fails.

這部分描述了使用 PostgreSQL 的用戶端程式介面。這些章節中的每一章都可以獨立閱讀。請注意，用戶端程序還有許多其他程式介面，這些程式介面獨立發佈並包含自己的文件（附錄 H 列出了一些較受歡迎的文件）。這部分的讀者應該熟悉使用 SQL 指令來操作和查詢資料庫（請參閱第 II 部分），當然還有程序語言的介面使用方式。

libpq 是 C 應用程式設計師對 PostgreSQL 的介面。libpq 是一組函式庫，提供用戶端程序可以將查詢傳遞給 PostgreSQL 後端伺務器並接收這些查詢的結果。

libpq 也是其他幾個 PostgreSQL 應用程序接口的底層引擎，包括為 C++、Perl、Python、Tcl 和 ECPG 程式的介面。因此，如果您使用其中一個軟體套件，libpq 行為的某些方面對您很重要。特別是，第 33.14 節，第 33.15 節和第 33.18 節描述了使用 libpq 應用程式的用戶可見行為。

本章末尾包含一些簡短的程式（第 33.21 節），以展示如何撰寫使用 libpq 的程式。在原始碼的目錄 src/test/examples 中還有幾個完整的 libpq 應用程式範例。

使用 libpq 的用戶端程式必須包含標頭檔 libpq-fe.h，並且必須與 libpq 函式庫連接。

These functions control miscellaneous details of libpq's behavior.PQclientEncoding

Returns the client encoding.

int PQclientEncoding(const PGconn *conn);

Note that it returns the encoding ID, not a symbolic string such as EUC_JP. If unsuccessful, it returns -1. To convert an encoding ID to an encoding name, you can use:

char *pg_encoding_to_char(int encoding_id);

PQsetClientEncoding

Sets the client encoding.

int PQsetClientEncoding(PGconn *conn, const char *encoding);

conn is a connection to the server, and encoding is the encoding you want to use. If the function successfully sets the encoding, it returns 0, otherwise -1. The current encoding for this connection can be determined by using PQclientEncoding.PQsetErrorVerbosity

Determines the verbosity of messages returned by PQerrorMessage and PQresultErrorMessage.

typedef enum
{
    PQERRORS_TERSE,
    PQERRORS_DEFAULT,
    PQERRORS_VERBOSE
} PGVerbosity;

PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity);

PQsetErrorVerbosity sets the verbosity mode, returning the connection's previous setting. In TERSE mode, returned messages include severity, primary text, and position only; this will normally fit on a single line. The default mode produces messages that include the above plus any detail, hint, or context fields (these might span multiple lines). The VERBOSE mode includes all available fields. Changing the verbosity does not affect the messages available from already-existing PGresult objects, only subsequently-created ones. (But see PQresultVerboseErrorMessage if you want to print a previous error with a different verbosity.)PQsetErrorContextVisibility

Determines the handling of CONTEXT fields in messages returned by PQerrorMessage and PQresultErrorMessage.

typedef enum
{
    PQSHOW_CONTEXT_NEVER,
    PQSHOW_CONTEXT_ERRORS,
    PQSHOW_CONTEXT_ALWAYS
} PGContextVisibility;

PGContextVisibility PQsetErrorContextVisibility(PGconn *conn, PGContextVisibility show_context);

PQsetErrorContextVisibility sets the context display mode, returning the connection's previous setting. This mode controls whether the CONTEXT field is included in messages (unless the verbosity setting is TERSE, in which case CONTEXT is never shown). The NEVER mode never includes CONTEXT, while ALWAYS always includes it if available. In ERRORS mode (the default), CONTEXT fields are included only for error messages, not for notices and warnings. Changing this mode does not affect the messages available from already-existing PGresult objects, only subsequently-created ones. (But see PQresultVerboseErrorMessage if you want to print a previous error with a different display mode.)PQtrace

Enables tracing of the client/server communication to a debugging file stream.

void PQtrace(PGconn *conn, FILE *stream);

Note

On Windows, if the libpq library and an application are compiled with different flags, this function call will crash the application because the internal representation of the FILEpointers differ. Specifically, multithreaded/single-threaded, release/debug, and static/dynamic flags should be the same for the library and all applications using that library.PQuntrace

Disables tracing started by PQtrace.

void PQuntrace(PGconn *conn);

這些函數可用於查詢現有資料庫連線物件的狀態。

提醒 libpq 應用程式設計師應該小心地使用 PGconn 的功能。使用下面描述的函數功能以獲取 PGconn 的內容。建議不要使用 libpq-int.h 引用內部 PGconn 變數，因為它們將來可能會有所改變。

以下函數回傳在連線建立時的參數值。這些值在連線的生命週期之間是不變的。如果使用多主機連線字串，則在使用同樣的 PGconn 物件建立新連線時，PQhost，PQport 和 PQpass 的值可能會變動。其他值在 PGconn 物件的生命週期內是不變的。

PQdb

回傳連線的資料庫名稱。

char *PQdb(const PGconn *conn);

PQuser

回傳連線的使用者名稱。

char *PQuser(const PGconn *conn);

PQpass

回傳連線所使用的密碼。

char *PQpass(const PGconn *conn);

PQhost

回傳連線的伺服器主機名稱。如果連線是透過 Unix-domain socket，則可以是主機名稱，IP 位址或目錄路徑。（路徑會區分大小寫，因為它始終是一個絕對路徑，並以 / 開頭。）

char *PQhost(const PGconn *conn);

PQport

回傳連線的連接埠號碼。

char *PQport(const PGconn *conn);

PQtty

回傳連線的除錯 TTY。（這已經過時了，因為伺服器不再關注 TTY 設定，但此功能仍然存在是為了相容性。）

char *PQtty(const PGconn *conn);

PQoptions

回傳連線請求中傳遞的命令列選項。

char *PQoptions(const PGconn *conn);

以下函數回傳狀態資訊，這些資訊會在 PGconn 物件上執行操作時變動。

PQstatus

回傳連線的狀態。

ConnStatusType PQstatus(const PGconn *conn);

狀態可以是許多值中的一個。但是，在非同步連線過程之外只能看到其中的兩個：CONNECTION_OK 和 CONNECTION_BAD。與資料庫的良好連線時是 CONNECTION_OK 狀態。CONNECTION_BAD 狀態表示連線嘗試失敗。通常情況下，OK 狀態將保持不變，直到 PQfinish，但網路故障可能導致狀態提前更改為 CONNECTION_BAD。在這種情況下，應用程式可以嘗試透過呼叫 PQreset 進行恢復。

有關可能回傳的其他狀態代碼，請參閱 PQconnectStartParams，PQconnectStart 和 PQconnectPoll 的項目。

PQtransactionStatus

回傳伺服器的目前在交易事務中狀態。

PGTransactionStatusType PQtransactionStatus(const PGconn *conn);

狀態可以是 PQTRANS_IDLE（目前空閒），PQTRANS_ACTIVE（命令正在進行中），PQTRANS_INTRANS（空閒，在有效的事務區塊中）或PQTRANS_INERROR（空閒，在失敗的事務區塊中）。 如果連線錯誤，則回報 PQTRANS_UNKNOWN。僅當查詢已發送到伺服器但尚未完成時才會回報 PQTRANS_ACTIVE。

PQparameterStatus

查詢伺服器的目前參數設定。

const char *PQparameterStatus(const PGconn *conn, const char *paramName);

伺服器在連線啟動時或其值發生變化時會自動回報某些參數值。PQparameterStatus 可用於查詢這些設定。如果參數已知，則回傳參數的目前值；如果參數未知，則回傳 NULL。

截至目前版本可回報的參數包括 server_version，server_encoding，client_encoding，application_name，is_superuser，session_authorization，DateStyle，IntervalStyle，TimeZone，integer_datetimes 和 standard_conforming_strings。（8.0 之前的版本沒有 server_encoding，TimeZone 和 integer_datetimes；8.1 之前的版本沒有 standard_conforming_strings；8.4 之前的版本沒有 IntervalStyle；9.0 之前的版本沒有 application_name。）請注意 server_version，server_encoding 和 integer_datetimes 啟動後無法更改。

3.0 之前的協定，伺服器不回報參數設定，但 libpq 包括了無論如何都要取得 server_version 和 client_encoding 的值的邏輯，所以鼓勵應用程式使用 PQparameterStatus 而不是直接的程式碼來確定這些值。（請注意，在 3.0 之前的連線上，在連線啟動後透過 SET 更改 client_encoding 將不會為 PQparameterStatus 所反映。）對於 server_version，另請參閱 PQserverVersion，它以更容易比較的數字形式回傳資訊。

如果沒有回報 standard_conforming_strings 的值，則應用程式可以假設它已經關閉；而倒斜線在字串文字中被視為轉置符號。此外，可以將此參數的存在視為接受轉置字串語法（E'...'）的指示。

雖然回傳的指標被宣告為 const，但它實際上指向與 PGconn 結構相關聯的可變的儲存空間。假設指標在查詢中保持有效是不明智的。

PQprotocolVersion

詢問正在使用的前端/後端協定。

int PQprotocolVersion(const PGconn *conn);

應用程式可能希望使用此函數來確定是否支援某些功能。目前，可能的值是 2（2.0 協議），3（3.0 協議）或零（連線錯誤）。 連線啟動完成後協定的版本不會改變，但理論上它可以在連線重置期間改變。通常在與 PostgreSQL 7.4 或更高版本的伺服器連線時使用 3.0 協議；7.4 之前的伺服器僅支持協定 2.0。（協定 1.0 已過時，libpq 不支援。）

PQserverVersion

回傳伺服器版本的整數表示。

int PQserverVersion(const PGconn *conn);

應用程式可以使用此函數來確定它們所連線的資料庫伺服器版本。結果是以伺服器的主要版本號乘以 10000 再加上次要版本號所組成的。例如，版本 10.1 將回傳為 100001，版本 11.0 將回傳為 110000。如果連線錯誤，則回傳零。

在主要版本 10 之前，PostgreSQL 使用三個部分的版本號碼，其中前兩個部分一起代表主要版本。對於這些版本，PQserverVersion 對每個部分使用兩位數字；例如，版本 9.1.5 將回傳為 90105，版本 9.2.0 將回傳為 90200。

因此，為了符合功能相容性，應用程式應將 PQserverVersion 的結果除以 100 而不是 10000，以確定主版本號碼。在所有版本系列中，只有最後兩位數字在次要版本（錯誤修復版本）之間有所不同。

PQerrorMessage

回傳最近於連線操作產生的錯誤訊息。

char *PQerrorMessage(const PGconn *conn);

如果 PQerrorMessage 失敗，幾乎所有的 libpq 函數都會為它們設定一條訊息。請注意，根據 libpq 的規則，非空的 PQerrorMessage 結果可以包含多行，並且將包含最後的換行符號。呼叫者不應該直接輸出結果。當關聯的 PGconn 物件傳遞給 PQfinish 時，它將被釋放。不應期望結果字串在 PGconn 結構的操作中保持相同。

PQsocket

Obtains the file descriptor number of the connection socket to the server. A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. (This will not change during normal operation, but could change during connection setup or reset.)

int PQsocket(const PGconn *conn);

PQbackendPID

Returns the process ID (PID) of the backend process handling this connection.

int PQbackendPID(const PGconn *conn);

The backend PID is useful for debugging purposes and for comparison to NOTIFY messages (which include the PID of the notifying backend process). Note that the PID belongs to a process executing on the database server host, not the local host!

PQconnectionNeedsPassword

Returns true (1) if the connection authentication method required a password, but none was available. Returns false (0) if not.

int PQconnectionNeedsPassword(const PGconn *conn);

This function can be applied after a failed connection attempt to decide whether to prompt the user for a password.

PQconnectionUsedPassword

Returns true (1) if the connection authentication method used a password. Returns false (0) if not.

int PQconnectionUsedPassword(const PGconn *conn);

This function can be applied after either a failed or successful connection attempt to detect whether the server demanded a password.

The following functions return information related to SSL. This information usually doesn't change after a connection is established.

PQsslInUse

Returns true (1) if the connection uses SSL, false (0) if not.

int PQsslInUse(const PGconn *conn);

PQsslAttribute

Returns SSL-related information about the connection.

const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);

The list of available attributes varies depending on the SSL library being used, and the type of connection. If an attribute is not available, returns NULL.

The following attributes are commonly available:

library

Name of the SSL implementation in use. (Currently, only "OpenSSL" is implemented)

protocol

SSL/TLS version in use. Common values are "TLSv1", "TLSv1.1" and "TLSv1.2", but an implementation may return other strings if some other protocol is used.

key_bits

Number of key bits used by the encryption algorithm.

cipher

A short name of the ciphersuite used, e.g. "DHE-RSA-DES-CBC3-SHA". The names are specific to each SSL implementation.

compression

If SSL compression is in use, returns the name of the compression algorithm, or "on" if compression is used but the algorithm is not known. If compression is not in use, returns "off".

PQsslAttributeNames

Return an array of SSL attribute names available. The array is terminated by a NULL pointer.

const char * const * PQsslAttributeNames(const PGconn *conn);

PQsslStruct

Return a pointer to an SSL-implementation-specific object describing the connection.

void *PQsslStruct(const PGconn *conn, const char *struct_name);

The struct(s) available depend on the SSL implementation in use. For OpenSSL, there is one struct, available under the name "OpenSSL", and it returns a pointer to the OpenSSL SSL struct. To use this function, code along the following lines could be used:

#include <libpq-fe.h>
#include <openssl/ssl.h>

...

    SSL *ssl;

    dbconn = PQconnectdb(...);
    ...

    ssl = PQsslStruct(dbconn, "OpenSSL");
    if (ssl)
    {
        /* use OpenSSL functions to access ssl */
    }

This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure.PQgetssl

Returns the SSL structure used in the connection, or null if SSL is not in use.

void *PQgetssl(const PGconn *conn);

This function is equivalent to PQsslStruct(conn, "OpenSSL"). It should not be used in new applications, because the returned struct is specific to OpenSSL and will not be available if another SSL implementation is used. To check if a connection uses SSL, call PQsslInUse instead, and for more details about the connection, use PQsslAttribute.

Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.

33.3.1. Main Functions

PQexec

Submits a command to the server and waits for the result.

PGresult *PQexec(PGconn *conn, const char *command);

Returns a PGresult pointer or possibly a null pointer. A non-null pointer will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the server. The PQresultStatus function should be called to check the return value for any errors (including the value of a null pointer, in which case it will return PGRES_FATAL_ERROR). Use PQerrorMessage to get more information about such errors.

The command string can include multiple SQL commands (separated by semicolons). Multiple queries sent in a single PQexec call are processed in a single transaction, unless there are explicit BEGIN/COMMIT commands included in the query string to divide it into multiple transactions. Note however that the returned PGresult structure describes only the result of the last command executed from the string. Should one of the commands fail, processing of the string stops with it and the returned PGresult describes the error condition.PQexecParams

Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text.

PGresult *PQexecParams(PGconn *conn,
                       const char *command,
                       int nParams,
                       const Oid *paramTypes,
                       const char * const *paramValues,
                       const int *paramLengths,
                       const int *paramFormats,
                       int resultFormat);

PQexecParams is like PQexec, but offers additional functionality: parameter values can be specified separately from the command string proper, and query results can be requested in either text or binary format. PQexecParams is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The function arguments are:conn

The connection object to send the command through.command

The SQL command string to be executed. If parameters are used, they are referred to in the command string as $1, $2, etc.nParams

The number of parameters supplied; it is the length of the arrays paramTypes[], paramValues[], paramLengths[], and paramFormats[]. (The array pointers can be NULL when nParams is zero.)paramTypes[]

Specifies, by OID, the data types to be assigned to the parameter symbols. If paramTypes is NULL, or any particular element in the array is zero, the server infers a data type for the parameter symbol in the same way it would do for an untyped literal string.paramValues[]

Specifies the actual values of the parameters. A null pointer in this array means the corresponding parameter is null; otherwise the pointer points to a zero-terminated text string (for text format) or binary data in the format expected by the server (for binary format).paramLengths[]

Specifies the actual data lengths of binary-format parameters. It is ignored for null parameters and text-format parameters. The array pointer can be null when there are no binary parameters.paramFormats[]

Specifies whether parameters are text (put a zero in the array entry for the corresponding parameter) or binary (put a one in the array entry for the corresponding parameter). If the array pointer is null then all parameters are presumed to be text strings.

Values passed in binary format require knowledge of the internal representation expected by the backend. For example, integers must be passed in network byte order. Passing numeric values requires knowledge of the server storage format, as implemented in src/backend/utils/adt/numeric.c::numeric_send() and src/backend/utils/adt/numeric.c::numeric_recv().resultFormat

Specify zero to obtain results in text format, or one to obtain results in binary format. (There is not currently a provision to obtain different result columns in different formats, although that is possible in the underlying protocol.)

The primary advantage of PQexecParams over PQexec is that parameter values can be separated from the command string, thus avoiding the need for tedious and error-prone quoting and escaping.

Unlike PQexec, PQexecParams allows at most one SQL command in the given string. (There can be semicolons in it, but not more than one nonempty command.) This is a limitation of the underlying protocol, but has some usefulness as an extra defense against SQL-injection attacks.

Tip

Specifying parameter types via OIDs is tedious, particularly if you prefer not to hard-wire particular OID values into your program. However, you can avoid doing so even in cases where the server by itself cannot determine the type of the parameter, or chooses a different type than you want. In the SQL command text, attach an explicit cast to the parameter symbol to show what data type you will send. For example:

SELECT * FROM mytable WHERE x = $1::bigint;

This forces parameter $1 to be treated as bigint, whereas by default it would be assigned the same type as x. Forcing the parameter type decision, either this way or by specifying a numeric type OID, is strongly recommended when sending parameter values in binary format, because binary format has less redundancy than text format and so there is less chance that the server will detect a type mismatch mistake for you.PQprepare

Submits a request to create a prepared statement with the given parameters, and waits for completion.

PGresult *PQprepare(PGconn *conn,
                    const char *stmtName,
                    const char *query,
                    int nParams,
                    const Oid *paramTypes);

PQprepare creates a prepared statement for later execution with PQexecPrepared. This feature allows commands to be executed repeatedly without being parsed and planned each time; see PREPARE for details. PQprepare is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The function creates a prepared statement named stmtName from the query string, which must contain a single SQL command. stmtName can be "" to create an unnamed statement, in which case any pre-existing unnamed statement is automatically replaced; otherwise it is an error if the statement name is already defined in the current session. If any parameters are used, they are referred to in the query as $1, $2, etc. nParams is the number of parameters for which types are pre-specified in the array paramTypes[]. (The array pointer can be NULL when nParams is zero.) paramTypes[] specifies, by OID, the data types to be assigned to the parameter symbols. If paramTypes is NULL, or any particular element in the array is zero, the server assigns a data type to the parameter symbol in the same way it would do for an untyped literal string. Also, the query can use parameter symbols with numbers higher than nParams; data types will be inferred for these symbols as well. (See PQdescribePrepared for a means to find out what data types were inferred.)

As with PQexec, the result is normally a PGresult object whose contents indicate server-side success or failure. A null result indicates out-of-memory or inability to send the command at all. Use PQerrorMessage to get more information about such errors.

Prepared statements for use with PQexecPrepared can also be created by executing SQL PREPARE statements. Also, although there is no libpq function for deleting a prepared statement, the SQL DEALLOCATE statement can be used for that purpose.PQexecPrepared

Sends a request to execute a prepared statement with given parameters, and waits for the result.

PGresult *PQexecPrepared(PGconn *conn,
                         const char *stmtName,
                         int nParams,
                         const char * const *paramValues,
                         const int *paramLengths,
                         const int *paramFormats,
                         int resultFormat);

PQexecPrepared is like PQexecParams, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. This feature allows commands that will be used repeatedly to be parsed and planned just once, rather than each time they are executed. The statement must have been prepared previously in the current session. PQexecPrepared is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The parameters are identical to PQexecParams, except that the name of a prepared statement is given instead of a query string, and the paramTypes[] parameter is not present (it is not needed since the prepared statement's parameter types were determined when it was created).PQdescribePrepared

Submits a request to obtain information about the specified prepared statement, and waits for completion.

PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);

PQdescribePrepared allows an application to obtain information about a previously prepared statement. PQdescribePrepared is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

stmtName can be "" or NULL to reference the unnamed statement, otherwise it must be the name of an existing prepared statement. On success, a PGresult with status PGRES_COMMAND_OK is returned. The functions PQnparams and PQparamtype can be applied to this PGresult to obtain information about the parameters of the prepared statement, and the functions PQnfields, PQfname, PQftype, etc provide information about the result columns (if any) of the statement.PQdescribePortal

Submits a request to obtain information about the specified portal, and waits for completion.

PGresult *PQdescribePortal(PGconn *conn, const char *portalName);

PQdescribePortal allows an application to obtain information about a previously created portal. (libpq does not provide any direct access to portals, but you can use this function to inspect the properties of a cursor created with a DECLARE CURSOR SQL command.) PQdescribePortal is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

portalName can be "" or NULL to reference the unnamed portal, otherwise it must be the name of an existing portal. On success, a PGresult with status PGRES_COMMAND_OK is returned. The functions PQnfields, PQfname, PQftype, etc can be applied to the PGresult to obtain information about the result columns (if any) of the portal.

The PGresult structure encapsulates the result returned by the server. libpq application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future.PQresultStatus

Returns the result status of the command.

ExecStatusType PQresultStatus(const PGresult *res);

PQresultStatus can return one of the following values:PGRES_EMPTY_QUERY

The string sent to the server was empty.PGRES_COMMAND_OK

Successful completion of a command returning no data.PGRES_TUPLES_OK

Successful completion of a command returning data (such as a SELECT or SHOW).PGRES_COPY_OUT

Copy Out (from server) data transfer started.PGRES_COPY_IN

Copy In (to server) data transfer started.PGRES_BAD_RESPONSE

The server's response was not understood.PGRES_NONFATAL_ERROR

A nonfatal error (a notice or warning) occurred.PGRES_FATAL_ERROR

A fatal error occurred.PGRES_COPY_BOTH

Copy In/Out (to and from server) data transfer started. This feature is currently used only for streaming replication, so this status should not occur in ordinary applications.PGRES_SINGLE_TUPLE

The PGresult contains a single result tuple from the current command. This status occurs only when single-row mode has been selected for the query (see Section 33.5).

If the result status is PGRES_TUPLES_OK or PGRES_SINGLE_TUPLE, then the functions described below can be used to retrieve the rows returned by the query. Note that a SELECTcommand that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT or UPDATE without a RETURNINGclause, etc.). A response of PGRES_EMPTY_QUERY might indicate a bug in the client software.

A result of status PGRES_NONFATAL_ERROR will never be returned directly by PQexec or other query execution functions; results of this kind are instead passed to the notice processor (see Section 33.12).PQresStatus

Converts the enumerated type returned by PQresultStatus into a string constant describing the status code. The caller should not free the result.

char *PQresStatus(ExecStatusType status);

PQresultErrorMessage

Returns the error message associated with the command, or an empty string if there was no error.

char *PQresultErrorMessage(const PGresult *res);

If there was an error, the returned string will include a trailing newline. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.

Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will return the same string as PQresultErrorMessage (on the result). However, a PGresultwill retain its error message until destroyed, whereas the connection's error message will change when subsequent operations are done. Use PQresultErrorMessage when you want to know the status associated with a particular PGresult; use PQerrorMessage when you want to know the status from the latest operation on the connection.PQresultVerboseErrorMessage

Returns a reformatted version of the error message associated with a PGresult object.

char *PQresultVerboseErrorMessage(const PGresult *res,
                                  PGVerbosity verbosity,
                                  PGContextVisibility show_context);

In some situations a client might wish to obtain a more detailed version of a previously-reported error. PQresultVerboseErrorMessage addresses this need by computing the message that would have been produced by PQresultErrorMessage if the specified verbosity settings had been in effect for the connection when the given PGresult was generated. If the PGresult is not an error result, “PGresult is not an error result” is reported instead. The returned string includes a trailing newline.

Unlike most other functions for extracting data from a PGresult, the result of this function is a freshly allocated string. The caller must free it using PQfreemem() when the string is no longer needed.

A NULL return is possible if there is insufficient memory.PQresultErrorField

Returns an individual field of an error report.

char *PQresultErrorField(const PGresult *res, int fieldcode);

fieldcode is an error field identifier; see the symbols listed below. NULL is returned if the PGresult is not an error or warning result, or does not include the specified field. Field values will normally not include a trailing newline. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.

The following field codes are available:PG_DIAG_SEVERITY

The severity; the field contents are ERROR, FATAL, or PANIC (in an error message), or WARNING, NOTICE, DEBUG, INFO, or LOG (in a notice message), or a localized translation of one of these. Always present.PG_DIAG_SEVERITY_NONLOCALIZED

The severity; the field contents are ERROR, FATAL, or PANIC (in an error message), or WARNING, NOTICE, DEBUG, INFO, or LOG (in a notice message). This is identical to the PG_DIAG_SEVERITY field except that the contents are never localized. This is present only in reports generated by PostgreSQL versions 9.6 and later.PG_DIAG_SQLSTATE

The SQLSTATE code for the error. The SQLSTATE code identifies the type of error that has occurred; it can be used by front-end applications to perform specific operations (such as error handling) in response to a particular database error. For a list of the possible SQLSTATE codes, see Appendix A. This field is not localizable, and is always present.PG_DIAG_MESSAGE_PRIMARY

The primary human-readable error message (typically one line). Always present.PG_DIAG_MESSAGE_DETAIL

Detail: an optional secondary error message carrying more detail about the problem. Might run to multiple lines.PG_DIAG_MESSAGE_HINT

Hint: an optional suggestion what to do about the problem. This is intended to differ from detail in that it offers advice (potentially inappropriate) rather than hard facts. Might run to multiple lines.PG_DIAG_STATEMENT_POSITION

A string containing a decimal integer indicating an error cursor position as an index into the original statement string. The first character has index 1, and positions are measured in characters not bytes.PG_DIAG_INTERNAL_POSITION

This is defined the same as the PG_DIAG_STATEMENT_POSITION field, but it is used when the cursor position refers to an internally generated command rather than the one submitted by the client. The PG_DIAG_INTERNAL_QUERY field will always appear when this field appears.PG_DIAG_INTERNAL_QUERY

The text of a failed internally-generated command. This could be, for example, a SQL query issued by a PL/pgSQL function.PG_DIAG_CONTEXT

An indication of the context in which the error occurred. Presently this includes a call stack traceback of active procedural language functions and internally-generated queries. The trace is one entry per line, most recent first.PG_DIAG_SCHEMA_NAME

If the error was associated with a specific database object, the name of the schema containing that object, if any.PG_DIAG_TABLE_NAME

If the error was associated with a specific table, the name of the table. (Refer to the schema name field for the name of the table's schema.)PG_DIAG_COLUMN_NAME

If the error was associated with a specific table column, the name of the column. (Refer to the schema and table name fields to identify the table.)PG_DIAG_DATATYPE_NAME

If the error was associated with a specific data type, the name of the data type. (Refer to the schema name field for the name of the data type's schema.)PG_DIAG_CONSTRAINT_NAME

If the error was associated with a specific constraint, the name of the constraint. Refer to fields listed above for the associated table or domain. (For this purpose, indexes are treated as constraints, even if they weren't created with constraint syntax.)PG_DIAG_SOURCE_FILE

The file name of the source-code location where the error was reported.PG_DIAG_SOURCE_LINE

The line number of the source-code location where the error was reported.PG_DIAG_SOURCE_FUNCTION

The name of the source-code function reporting the error.

Note

The fields for schema name, table name, column name, data type name, and constraint name are supplied only for a limited number of error types; see Appendix A. Do not assume that the presence of any of these fields guarantees the presence of another field. Core error sources observe the interrelationships noted above, but user-defined functions may use these fields in other ways. In the same vein, do not assume that these fields denote contemporary objects in the current database.

The client is responsible for formatting displayed information to meet its needs; in particular it should break long lines as needed. Newline characters appearing in the error message fields should be treated as paragraph breaks, not line breaks.

Errors generated internally by libpq will have severity and primary message, but typically no other fields. Errors returned by a pre-3.0-protocol server will include severity and primary message, and sometimes a detail message, but no other fields.

Note that error fields are only available from PGresult objects, not PGconn objects; there is no PQerrorField function.PQclear

Frees the storage associated with a PGresult. Every command result should be freed via PQclear when it is no longer needed.

void PQclear(PGresult *res);

You can keep a PGresult object around for as long as you need it; it does not go away when you issue a new command, nor even if you close the connection. To get rid of it, you must call PQclear. Failure to do this will result in memory leaks in your application.

33.3.2. Retrieving Query Result Information

These functions are used to extract information from a PGresult object that represents a successful query result (that is, one that has status PGRES_TUPLES_OK or PGRES_SINGLE_TUPLE). They can also be used to extract information from a successful Describe operation: a Describe's result has all the same column information that actual execution of the query would provide, but it has zero rows. For objects with other status values, these functions will act as though the result has zero rows and zero columns.PQntuples

Returns the number of rows (tuples) in the query result. (Note that PGresult objects are limited to no more than INT_MAX rows, so an int result is sufficient.)

int PQntuples(const PGresult *res);

PQnfields

Returns the number of columns (fields) in each row of the query result.

int PQnfields(const PGresult *res);

PQfname

Returns the column name associated with the given column number. Column numbers start at 0. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.

char *PQfname(const PGresult *res,
              int column_number);

NULL is returned if the column number is out of range.PQfnumber

Returns the column number associated with the given column name.

int PQfnumber(const PGresult *res,
              const char *column_name);

-1 is returned if the given name does not match any column.

The given name is treated like an identifier in an SQL command, that is, it is downcased unless double-quoted. For example, given a query result generated from the SQL command:

SELECT 1 AS FOO, 2 AS "BAR";

we would have the results:

PQfname(res, 0)              foo
PQfname(res, 1)              BAR
PQfnumber(res, "FOO")        0
PQfnumber(res, "foo")        0
PQfnumber(res, "BAR")        -1
PQfnumber(res, "\"BAR\"")    1

PQftable

Returns the OID of the table from which the given column was fetched. Column numbers start at 0.

Oid PQftable(const PGresult *res,
             int column_number);

InvalidOid is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol. You can query the system table pg_class to determine exactly which table is referenced.

The type Oid and the constant InvalidOid will be defined when you include the libpq header file. They will both be some integer type.PQftablecol

Returns the column number (within its table) of the column making up the specified query result column. Query-result column numbers start at 0, but table columns have nonzero numbers.

int PQftablecol(const PGresult *res,
                int column_number);

Zero is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol.PQfformat

Returns the format code indicating the format of the given column. Column numbers start at 0.

int PQfformat(const PGresult *res,
              int column_number);

Format code zero indicates textual data representation, while format code one indicates binary representation. (Other codes are reserved for future definition.)PQftype

Returns the data type associated with the given column number. The integer returned is the internal OID number of the type. Column numbers start at 0.

Oid PQftype(const PGresult *res,
            int column_number);

You can query the system table pg_type to obtain the names and properties of the various data types. The OIDs of the built-in data types are defined in the file src/include/catalog/pg_type.h in the source tree.PQfmod

Returns the type modifier of the column associated with the given column number. Column numbers start at 0.

int PQfmod(const PGresult *res,
           int column_number);

The interpretation of modifier values is type-specific; they typically indicate precision or size limits. The value -1 is used to indicate “no information available”. Most data types do not use modifiers, in which case the value is always -1.PQfsize

Returns the size in bytes of the column associated with the given column number. Column numbers start at 0.

int PQfsize(const PGresult *res,
            int column_number);

PQfsize returns the space allocated for this column in a database row, in other words the size of the server's internal representation of the data type. (Accordingly, it is not really very useful to clients.) A negative value indicates the data type is variable-length.PQbinaryTuples

Returns 1 if the PGresult contains binary data and 0 if it contains text data.

int PQbinaryTuples(const PGresult *res);

This function is deprecated (except for its use in connection with COPY), because it is possible for a single PGresult to contain text data in some columns and binary data in others. PQfformat is preferred. PQbinaryTuples returns 1 only if all columns of the result are binary (format 1).PQgetvalue

Returns a single field value of one row of a PGresult. Row and column numbers start at 0. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.

char *PQgetvalue(const PGresult *res,
                 int row_number,
                 int column_number);

For data in text format, the value returned by PQgetvalue is a null-terminated character string representation of the field value. For data in binary format, the value is in the binary representation determined by the data type's typsend and typreceive functions. (The value is actually followed by a zero byte in this case too, but that is not ordinarily useful, since the value is likely to contain embedded nulls.)

An empty string is returned if the field value is null. See PQgetisnull to distinguish null values from empty-string values.

The pointer returned by PQgetvalue points to storage that is part of the PGresult structure. One should not modify the data it points to, and one must explicitly copy the data into other storage if it is to be used past the lifetime of the PGresult structure itself.PQgetisnull

Tests a field for a null value. Row and column numbers start at 0.

int PQgetisnull(const PGresult *res,
                int row_number,
                int column_number);

This function returns 1 if the field is null and 0 if it contains a non-null value. (Note that PQgetvalue will return an empty string, not a null pointer, for a null field.)PQgetlength

Returns the actual length of a field value in bytes. Row and column numbers start at 0.

int PQgetlength(const PGresult *res,
                int row_number,
                int column_number);

This is the actual data length for the particular data value, that is, the size of the object pointed to by PQgetvalue. For text data format this is the same as strlen(). For binary format this is essential information. Note that one should not rely on PQfsize to obtain the actual data length.PQnparams

Returns the number of parameters of a prepared statement.

int PQnparams(const PGresult *res);

This function is only useful when inspecting the result of PQdescribePrepared. For other types of queries it will return zero.PQparamtype

Returns the data type of the indicated statement parameter. Parameter numbers start at 0.

Oid PQparamtype(const PGresult *res, int param_number);

This function is only useful when inspecting the result of PQdescribePrepared. For other types of queries it will return zero.PQprint

Prints out all the rows and, optionally, the column names to the specified output stream.

void PQprint(FILE *fout,      /* output stream */
             const PGresult *res,
             const PQprintOpt *po);
typedef struct
{
    pqbool  header;      /* print output field headings and row count */
    pqbool  align;       /* fill align the fields */
    pqbool  standard;    /* old brain dead format */
    pqbool  html3;       /* output HTML tables */
    pqbool  expanded;    /* expand tables */
    pqbool  pager;       /* use pager for output if needed */
    char    *fieldSep;   /* field separator */
    char    *tableOpt;   /* attributes for HTML table element */
    char    *caption;    /* HTML table caption */
    char    **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;

This function was formerly used by psql to print query results, but this is no longer the case. Note that it assumes all the data is in text format.

33.3.3. Retrieving Other Result Information

These functions are used to extract other information from PGresult objects.PQcmdStatus

Returns the command status tag from the SQL command that generated the PGresult.

char *PQcmdStatus(PGresult *res);

Commonly this is just the name of the command, but it might include additional data such as the number of rows processed. The caller should not free the result directly. It will be freed when the associated PGresult handle is passed to PQclear.PQcmdTuples

Returns the number of rows affected by the SQL command.

char *PQcmdTuples(PGresult *res);

This function returns a string containing the number of rows affected by the SQL statement that generated the PGresult. This function can only be used following the execution of a SELECT, CREATE TABLE AS, INSERT, UPDATE, DELETE, MOVE, FETCH, or COPY statement, or an EXECUTE of a prepared query that contains an INSERT, UPDATE, or DELETE statement. If the command that generated the PGresult was anything else, PQcmdTuples returns an empty string. The caller should not free the return value directly. It will be freed when the associated PGresult handle is passed to PQclear.PQoidValue

Returns the OID of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs, or a EXECUTE of a prepared query containing a suitable INSERT statement. Otherwise, this function returns InvalidOid. This function will also return InvalidOid if the table affected by the INSERT statement does not contain OIDs.

Oid PQoidValue(const PGresult *res);

PQoidStatus

This function is deprecated in favor of PQoidValue and is not thread-safe. It returns a string with the OID of the inserted row, while PQoidValue returns the OID value.

char *PQoidStatus(const PGresult *res);

33.3.4. Escaping Strings for Inclusion in SQL Commands

PQescapeLiteral

char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);

PQescapeLiteral escapes a string for use within an SQL command. This is useful when inserting data values as literal constants in SQL commands. Certain characters (such as quotes and backslashes) must be escaped to prevent them from being interpreted specially by the SQL parser. PQescapeLiteral performs this operation.

PQescapeLiteral returns an escaped version of the str parameter in memory allocated with malloc(). This memory should be freed using PQfreemem() when the result is no longer needed. A terminating zero byte is not required, and should not be counted in length. (If a terminating zero byte is found before length bytes are processed, PQescapeLiteralstops at the zero; the behavior is thus rather like strncpy.) The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser. A terminating zero byte is also added. The single quotes that must surround PostgreSQL string literals are included in the result string.

On error, PQescapeLiteral returns NULL and a suitable message is stored in the conn object.

Tip

It is especially important to do proper escaping when handling strings that were received from an untrustworthy source. Otherwise there is a security risk: you are vulnerable to “SQL injection” attacks wherein unwanted SQL commands are fed to your database.

Note that it is not necessary nor correct to do escaping when a data value is passed as a separate parameter in PQexecParams or its sibling routines.PQescapeIdentifier

char *PQescapeIdentifier(PGconn *conn, const char *str, size_t length);

PQescapeIdentifier escapes a string for use as an SQL identifier, such as a table, column, or function name. This is useful when a user-supplied identifier might contain special characters that would otherwise not be interpreted as part of the identifier by the SQL parser, or when the identifier might contain upper case characters whose case should be preserved.

PQescapeIdentifier returns a version of the str parameter escaped as an SQL identifier in memory allocated with malloc(). This memory must be freed using PQfreemem() when the result is no longer needed. A terminating zero byte is not required, and should not be counted in length. (If a terminating zero byte is found before length bytes are processed, PQescapeIdentifier stops at the zero; the behavior is thus rather like strncpy.) The return string has all special characters replaced so that it will be properly processed as an SQL identifier. A terminating zero byte is also added. The return string will also be surrounded by double quotes.

On error, PQescapeIdentifier returns NULL and a suitable message is stored in the conn object.

Tip

As with string literals, to prevent SQL injection attacks, SQL identifiers must be escaped when they are received from an untrustworthy source.PQescapeStringConn

size_t PQescapeStringConn(PGconn *conn,
                          char *to, const char *from, size_t length,
                          int *error);

PQescapeStringConn escapes string literals, much like PQescapeLiteral. Unlike PQescapeLiteral, the caller is responsible for providing an appropriately sized buffer. Furthermore, PQescapeStringConn does not generate the single quotes that must surround PostgreSQL string literals; they should be provided in the SQL command that the result is inserted into. The parameter from points to the first character of the string that is to be escaped, and the length parameter gives the number of bytes in this string. A terminating zero byte is not required, and should not be counted in length. (If a terminating zero byte is found before length bytes are processed, PQescapeStringConn stops at the zero; the behavior is thus rather like strncpy.) to shall point to a buffer that is able to hold at least one more byte than twice the value of length, otherwise the behavior is undefined. Behavior is likewise undefined if the to and from strings overlap.

If the error parameter is not NULL, then *error is set to zero on success, nonzero on error. Presently the only possible error conditions involve invalid multibyte encoding in the source string. The output string is still generated on error, but it can be expected that the server will reject it as malformed. On error, a suitable message is stored in the connobject, whether or not error is NULL.

PQescapeStringConn returns the number of bytes written to to, not including the terminating zero byte.PQescapeString

PQescapeString is an older, deprecated version of PQescapeStringConn.

size_t PQescapeString (char *to, const char *from, size_t length);

The only difference from PQescapeStringConn is that PQescapeString does not take PGconn or error parameters. Because of this, it cannot adjust its behavior depending on the connection properties (such as character encoding) and therefore it might give the wrong results. Also, it has no way to report error conditions.

PQescapeString can be used safely in client programs that work with only one PostgreSQL connection at a time (in this case it can find out what it needs to know “behind the scenes”). In other contexts it is a security hazard and should be avoided in favor of PQescapeStringConn.PQescapeByteaConn

Escapes binary data for use within an SQL command with the type bytea. As with PQescapeStringConn, this is only used when inserting data directly into an SQL command string.

unsigned char *PQescapeByteaConn(PGconn *conn,
                                 const unsigned char *from,
                                 size_t from_length,
                                 size_t *to_length);

Certain byte values must be escaped when used as part of a bytea literal in an SQL statement. PQescapeByteaConn escapes bytes using either hex encoding or backslash escaping. See Section 8.4 for more information.

The from parameter points to the first byte of the string that is to be escaped, and the from_length parameter gives the number of bytes in this binary string. (A terminating zero byte is neither necessary nor counted.) The to_length parameter points to a variable that will hold the resultant escaped string length. This result string length includes the terminating zero byte of the result.

PQescapeByteaConn returns an escaped version of the from parameter binary string in memory allocated with malloc(). This memory should be freed using PQfreemem() when the result is no longer needed. The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser, and the byteainput function. A terminating zero byte is also added. The single quotes that must surround PostgreSQL string literals are not part of the result string.

On error, a null pointer is returned, and a suitable error message is stored in the conn object. Currently, the only possible error is insufficient memory for the result string.PQescapeBytea

PQescapeBytea is an older, deprecated version of PQescapeByteaConn.

unsigned char *PQescapeBytea(const unsigned char *from,
                             size_t from_length,
                             size_t *to_length);

The only difference from PQescapeByteaConn is that PQescapeBytea does not take a PGconn parameter. Because of this, PQescapeBytea can only be used safely in client programs that use a single PostgreSQL connection at a time (in this case it can find out what it needs to know “behind the scenes”). It might give the wrong results if used in programs that use multiple database connections (use PQescapeByteaConn in such cases).PQunescapeBytea

Converts a string representation of binary data into binary data — the reverse of PQescapeBytea. This is needed when retrieving bytea data in text format, but not when retrieving it in binary format.

unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);

The from parameter points to a string such as might be returned by PQgetvalue when applied to a bytea column. PQunescapeBytea converts this string representation into its binary representation. It returns a pointer to a buffer allocated with malloc(), or NULL on error, and puts the size of the buffer in to_length. The result must be freed using PQfreememwhen it is no longer needed.

This conversion is not exactly the inverse of PQescapeBytea, because the string is not expected to be “escaped” when received from PQgetvalue. In particular this means there is no need for string quoting considerations, and so no need for a PGconn parameter.

The following functions deal with making a connection to a PostgreSQL backend server. An application program can have several backend connections open at one time. (One reason to do that is to access more than one database.) Each connection is represented by a PGconn object, which is obtained from the function PQconnectdb, PQconnectdbParams, or PQsetdbLogin. Note that these functions will always return a non-null object pointer, unless perhaps there is too little memory even to allocate the PGconn object. The PQstatus function should be called to check the return value for a successful connection before queries are sent via the connection object.

Warning

If untrusted users have access to a database that has not adopted a , begin each session by removing publicly-writable schemas from search_path. One can set parameter key word options to value -csearch_path=. Alternately, one can issue PQexec(conn, "SELECT pg_catalog.set_config('search_path', '', false)") after connecting. This consideration is not specific to libpq; it applies to every interface for executing arbitrary SQL commands.

Warning

On Unix, forking a process with open libpq connections can lead to unpredictable results because the parent and child processes share the same sockets and operating system resources. For this reason, such usage is not recommended, though doing an exec from the child process to load a new executable is safe.

Note

On Windows, there is a way to improve performance if a single database connection is repeatedly started and shutdown. Internally, libpq calls WSAStartup() and WSACleanup() for connection startup and shutdown, respectively. WSAStartup() increments an internal Windows library reference count which is decremented by WSACleanup(). When the reference count is just one, calling WSACleanup() frees all resources and all DLLs are unloaded. This is an expensive operation. To avoid this, an application can manually call WSAStartup() so resources will not be freed when the last database connection is closed.PQconnectdbParams

Makes a new connection to the database server.

This function opens a new database connection using the parameters taken from two NULL-terminated arrays. The first, keywords, is defined as an array of strings, each one being a key word. The second, values, gives the value for each key word. Unlike PQsetdbLogin below, the parameter set can be extended without changing the function signature, so use of this function (or its nonblocking analogs PQconnectStartParams and PQconnectPoll) is preferred for new application programming.

The currently recognized parameter key words are listed in .

When expand_dbname is non-zero, the dbname key word value is allowed to be recognized as a connection string. Only the first occurrence of dbname is expanded this way, any subsequent dbname value is processed as plain database name. More details on the possible connection string formats appear in .

The passed arrays can be empty to use all default parameters, or can contain one or more parameter settings. They should be matched in length. Processing will stop at the first NULL element in the keywords array.

If any parameter is NULL or an empty string, the corresponding environment variable (see ) is checked. If the environment variable is not set either, then the indicated built-in defaults are used.

In general key words are processed from the beginning of these arrays in index order. The effect of this is that when key words are repeated, the last processed value is retained. Therefore, through careful placement of the dbname key word, it is possible to determine what may be overridden by a conninfo string, and what may not.PQconnectdb

Makes a new connection to the database server.

This function opens a new database connection using the parameters taken from the string conninfo.

Makes a new connection to the database server.

This is the predecessor of PQconnectdb with a fixed set of parameters. It has the same functionality except that the missing parameters will always take on default values. Write NULL or an empty string for any one of the fixed parameters that is to be defaulted.

If the dbName contains an = sign or has a valid connection URI prefix, it is taken as a conninfo string in exactly the same way as if it had been passed to PQconnectdb, and the remaining parameters are then applied as specified for PQconnectdbParams.PQsetdb

Makes a new connection to the database server.

This is a macro that calls PQsetdbLogin with null pointers for the login and pwd parameters. It is provided for backward compatibility with very old programs.PQconnectStartParams PQconnectStart PQconnectPoll

Make a connection to the database server in a nonblocking manner.

These three functions are used to open a connection to a database server such that your application's thread of execution is not blocked on remote I/O whilst doing so. The point of this approach is that the waits for I/O to complete can occur in the application's main loop, rather than down inside PQconnectdbParams or PQconnectdb, and so the application can manage this operation in parallel with other activities.

With PQconnectStartParams, the database connection is made using the parameters taken from the keywords and values arrays, and controlled by expand_dbname, as described above for PQconnectdbParams.

With PQconnectStart, the database connection is made using the parameters taken from the string conninfo as described above for PQconnectdb.

Neither PQconnectStartParams nor PQconnectStart nor PQconnectPoll will block, so long as a number of restrictions are met:

• If you call PQtrace, ensure that the stream object into which you trace will not block.

• You must ensure that the socket is in the appropriate state before calling PQconnectPoll, as described below.

To begin a nonblocking connection request, call PQconnectStart or PQconnectStartParams. If the result is null, then libpq has been unable to allocate a new PGconn structure. Otherwise, a valid PGconn pointer is returned (though not yet representing a valid connection to the database). Next call PQstatus(conn). If the result is CONNECTION_BAD, the connection attempt has already failed, typically because of invalid connection parameters.

If PQconnectStart or PQconnectStartParams succeeds, the next stage is to poll libpq so that it can proceed with the connection sequence. Use PQsocket(conn) to obtain the descriptor of the socket underlying the database connection. (Caution: do not assume that the socket remains the same across PQconnectPoll calls.) Loop thus: If PQconnectPoll(conn) last returned PGRES_POLLING_READING, wait until the socket is ready to read (as indicated by select(), poll(), or similar system function). Then call PQconnectPoll(conn) again. Conversely, if PQconnectPoll(conn) last returned PGRES_POLLING_WRITING, wait until the socket is ready to write, then call PQconnectPoll(conn) again. On the first iteration, i.e. if you have yet to call PQconnectPoll, behave as if it last returned PGRES_POLLING_WRITING. Continue this loop until PQconnectPoll(conn) returns PGRES_POLLING_FAILED, indicating the connection procedure has failed, or PGRES_POLLING_OK, indicating the connection has been successfully made.

At any time during connection, the status of the connection can be checked by calling PQstatus. If this call returns CONNECTION_BAD, then the connection procedure has failed; if the call returns CONNECTION_OK, then the connection is ready. Both of these states are equally detectable from the return value of PQconnectPoll, described above. Other states might also occur during (and only during) an asynchronous connection procedure. These indicate the current stage of the connection procedure and might be useful to provide feedback to the user for example. These statuses are:CONNECTION_STARTED

Waiting for connection to be made.CONNECTION_MADE

Connection OK; waiting to send.CONNECTION_AWAITING_RESPONSE

Waiting for a response from the server.CONNECTION_AUTH_OK

Received authentication; waiting for backend start-up to finish.CONNECTION_SSL_STARTUP

Negotiating SSL encryption.CONNECTION_SETENV

Negotiating environment-driven parameter settings.CONNECTION_CHECK_WRITABLE

Checking if connection is able to handle write transactions.CONNECTION_CONSUME

Consuming any remaining response messages on connection.

Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these occurring in a particular order, or at all, or on the status always being one of these documented values. An application might do something like this:

The connect_timeout connection parameter is ignored when using PQconnectPoll; it is the application's responsibility to decide whether an excessive amount of time has elapsed. Otherwise, PQconnectStart followed by a PQconnectPoll loop is equivalent to PQconnectdb.

Note that when PQconnectStart or PQconnectStartParams returns a non-null pointer, you must call PQfinish when you are finished with it, in order to dispose of the structure and any associated memory blocks. This must be done even if the connection attempt fails or is abandoned.PQconndefaults

Returns the default connection options.

Returns a connection options array. This can be used to determine all possible PQconnectdb options and their current default values. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer. The null pointer is returned if memory could not be allocated. Note that the current default values (val fields) will depend on environment variables and other context. A missing or invalid service file will be silently ignored. Callers must treat the connection options data as read-only.

After processing the options array, free it by passing it to PQconninfoFree. If this is not done, a small amount of memory is leaked for each call to PQconndefaults.PQconninfo

Returns the connection options used by a live connection.

Returns a connection options array. This can be used to determine all possible PQconnectdb options and the values that were used to connect to the server. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer. All notes above for PQconndefaults also apply to the result of PQconninfo.PQconninfoParse

Returns parsed connection options from the provided connection string.

Parses a connection string and returns the resulting options as an array; or returns NULL if there is a problem with the connection string. This function can be used to extract the PQconnectdb options in the provided connection string. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer.

All legal options will be present in the result array, but the PQconninfoOption for any option not present in the connection string will have val set to NULL; default values are not inserted.

If errmsg is not NULL, then *errmsg is set to NULL on success, else to a malloc'd error string explaining the problem. (It is also possible for *errmsg to be set to NULL and the function to return NULL; this indicates an out-of-memory condition.)

After processing the options array, free it by passing it to PQconninfoFree. If this is not done, some memory is leaked for each call to PQconninfoParse. Conversely, if an error occurs and errmsg is not NULL, be sure to free the error string using PQfreemem.PQfinish

Closes the connection to the server. Also frees memory used by the PGconn object.

Note that even if the server connection attempt fails (as indicated by PQstatus), the application should call PQfinish to free the memory used by the PGconn object. The PGconn pointer must not be used again after PQfinish has been called.PQreset

Resets the communication channel to the server.

This function will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This might be useful for error recovery if a working connection is lost.PQresetStart PQresetPoll

Reset the communication channel to the server, in a nonblocking manner.

These functions will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This can be useful for error recovery if a working connection is lost. They differ from PQreset (above) in that they act in a nonblocking manner. These functions suffer from the same restrictions as PQconnectStartParams, PQconnectStart and PQconnectPoll.

To initiate a connection reset, call PQresetStart. If it returns 0, the reset has failed. If it returns 1, poll the reset using PQresetPoll in exactly the same way as you would create the connection using PQconnectPoll.

PQpingParams

PQpingParams reports the status of the server. It accepts connection parameters identical to those of PQconnectdbParams, described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.

The function returns one of the following values:

PQPING_OK

The server is running and appears to be accepting connections.

PQPING_REJECT

The server is running but is in a state that disallows connections (startup, shutdown, or crash recovery).

PQPING_NO_RESPONSE

The server could not be contacted. This might indicate that the server is not running, or that there is something wrong with the given connection parameters (for example, wrong port number), or that there is a network connectivity problem (for example, a firewall blocking the connection request).

PQPING_NO_ATTEMPT

No attempt was made to contact the server, because the supplied parameters were obviously incorrect or there was some client-side problem (for example, out of memory).

PQping

PQping reports the status of the server. It accepts connection parameters identical to those of PQconnectdb, described above. It is not necessary to supply correct user name, password, or database name values to obtain the server status; however, if incorrect values are provided, the server will log a failed connection attempt.

The return values are the same as for PQpingParams.

34.1.1. Connection Strings

34.1.1.1. Keyword/Value Connection Strings

In the first format, each parameter setting is in the form keyword = value. Spaces around the equal sign are optional. To write an empty value, or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., \' and \\.

Example:

34.1.1.2. Connection URIs

The general form for a connection URI is:

The URI scheme designator can be either postgresql:// or postgres://. Each of the URI parts is optional. The following examples illustrate valid URI syntax uses:

Components of the hierarchical part of the URI can also be given as parameters. For example:

Percent-encoding may be used to include symbols with special meaning in any of the URI parts, e.g. replace = with %3D.

For improved compatibility with JDBC connection URIs, instances of parameter ssl=true are translated into sslmode=require.

The host part may be either host name or an IP address. To specify an IPv6 host address, enclose it in square brackets:

It is possible to specify multiple host components, each with an optional port component, in a single URI. A URI of the form postgresql://host1:port1,host2:port2,host3:port3/ is equivalent to a connection string of the form host=host1,host2,host3 port=port1,port2,port3. Each host will be tried in turn until a connection is successfully established.

34.1.1.3. Specifying Multiple Hosts

It is possible to specify multiple hosts to connect to, so that they are tried in the given order. In the Keyword/Value format, the host, hostaddr, and port options accept a comma-separated list of values. The same number of elements must be given in each option that is specified, such that e.g. the first hostaddr corresponds to the first host name, the second hostaddr corresponds to the second host name, and so forth. As an exception, if only one port is specified, it applies to all the hosts.

In the connection URI format, you can list multiple host:port pairs separated by commas, in the host component of the URI.

In either format, a single host name can translate to multiple network addresses. A common example of this is a host that has both an IPv4 and an IPv6 address.

When multiple hosts are specified, or when a single host name is translated to multiple addresses, all the hosts and addresses will be tried in order, until one succeeds. If none of the hosts can be reached, the connection fails. If a connection is established successfully, but authentication fails, the remaining hosts in the list are not tried.

If a password file is used, you can have different passwords for different hosts. All the other connection options are the same for every host in the list; it is not possible to e.g. specify different usernames for different hosts.

34.1.2. Parameter Key Words

The currently recognized parameter key words are:host

Name of host to connect to. If a host name begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored. The default behavior when host is not specified, or is empty, is to connect to a Unix-domain socket in /tmp (or whatever socket directory was specified when PostgreSQL was built). On machines without Unix-domain sockets, the default is to connect to localhost.

hostaddr

Numeric IP address of host to connect to. This should be in the standard IPv4 address format, e.g., 172.28.40.9. If your machine supports IPv6, you can also use those addresses. TCP/IP communication is always used when a nonempty string is specified for this parameter.

Using hostaddr instead of host allows the application to avoid a host name look-up, which might be important in applications with time constraints. However, a host name is required for GSSAPI or SSPI authentication methods, as well as for verify-full SSL certificate verification. The following rules are used:

• If host is specified without hostaddr, a host name lookup occurs. (When using PQconnectPoll, the lookup occurs when PQconnectPoll first considers this host name, and it may cause PQconnectPoll to block for a significant amount of time.)

• If hostaddr is specified without host, the value for hostaddr gives the server network address. The connection attempt will fail if the authentication method requires a host name.

• If both host and hostaddr are specified, the value for hostaddr gives the server network address. The value for host is ignored unless the authentication method requires it, in which case it will be used as the host name.

Without either a host name or host address, libpq will connect using a local Unix-domain socket; or on machines without Unix-domain sockets, it will attempt to connect to localhost.

port

Port number to connect to at the server host, or socket file name extension for Unix-domain connections. If multiple hosts were given in the host or hostaddr parameters, this parameter may specify a comma-separated list of ports of the same length as the host list, or it may specify a single port number to be used for all hosts. An empty string, or an empty item in a comma-separated list, specifies the default port number established when PostgreSQL was built.

dbname

user

PostgreSQL user name to connect as. Defaults to be the same as the operating system name of the user running the application.

password

Password to be used if the server demands password authentication.

passfile

connect_timeout

Maximum wait for connection, in seconds (write as a decimal integer, e.g. 10). Zero, negative, or not specified means wait indefinitely. The minimum allowed timeout is 2 seconds, therefore a value of 1 is interpreted as 2. This timeout applies separately to each host name or IP address. For example, if you specify two hosts and connect_timeout is 5, each host will time out if no connection is made within 5 seconds, so the total time spent waiting for a connection might be up to 10 seconds.

client_encoding

This sets the client_encoding configuration parameter for this connection. In addition to the values accepted by the corresponding server option, you can use auto to determine the right encoding from the current locale in the client (LC_CTYPE environment variable on Unix systems).

options

fallback_application_name

keepalives

Controls whether client-side TCP keepalives are used. The default value is 1, meaning on, but you can change this to 0, meaning off, if keepalives are not wanted. This parameter is ignored for connections made via a Unix-domain socket.

keepalives_idle

Controls the number of seconds of inactivity after which TCP should send a keepalive message to the server. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where TCP_KEEPIDLE or an equivalent socket option is available, and on Windows; on other systems, it has no effect.

keepalives_interval

Controls the number of seconds after which a TCP keepalive message that is not acknowledged by the server should be retransmitted. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where TCP_KEEPINTVL or an equivalent socket option is available, and on Windows; on other systems, it has no effect.

keepalives_count

Controls the number of TCP keepalives that can be lost before the client's connection to the server is considered dead. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where TCP_KEEPCNT or an equivalent socket option is available; on other systems, it has no effect.

tty

Ignored (formerly, this specified where to send server debug output).

replication

The following values, which are case-insensitive, are supported:

true, on, yes, 1

The connection goes into physical replication mode.

database

The connection goes into logical replication mode, connecting to the database specified in the dbname parameter.

false, off, no, 0

The connection is a regular one, which is the default behavior.

In physical or logical replication mode, only the simple query protocol can be used.s

slmode

This option determines whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server. There are six modes:

disable

only try a non-SSL connectionallow

first try a non-SSL connection; if that fails, try an SSL connectionprefer (default)

first try an SSL connection; if that fails, try a non-SSL connectionrequire

only try an SSL connection. If a root CA file is present, verify the certificate in the same way as if verify-ca was specifiedverify-ca

only try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority (CA)verify-full

only try an SSL connection, verify that the server certificate is issued by a trusted CA and that the requested server host name matches that in the certificate

sslmode is ignored for Unix domain socket communication. If PostgreSQL is compiled without SSL support, using options require, verify-ca, or verify-full will cause an error, while options allow and prefer will be accepted but libpq will not actually attempt an SSL connection.

requiressl

This option is deprecated in favor of the sslmode setting.

If set to 1, an SSL connection to the server is required (this is equivalent to sslmode require). libpq will then refuse to connect if the server does not accept an SSL connection. If set to 0 (default), libpq will negotiate the connection type with the server (equivalent to sslmode prefer). This option is only available if PostgreSQL is compiled with SSL support.

sslcompression

If set to 1, data sent over SSL connections will be compressed. If set to 0, compression will be disabled. The default is 0. This parameter is ignored if a connection without SSL is made.

SSL compression is nowadays considered insecure and its use is no longer recommended. OpenSSL 1.1.0 disables compression by default, and many operating system distributions disable it in prior versions as well, so setting this parameter to on will not have any effect if the server does not accept compression. On the other hand, OpenSSL before 1.0.0 does not support disabling compression, so this parameter is ignored with those versions, and whether compression is used depends on the server.

If security is not a primary concern, compression can improve throughput if the network is the bottleneck. Disabling compression can improve response time and throughput if CPU performance is the limiting factor.

sslcert

This parameter specifies the file name of the client SSL certificate, replacing the default ~/.postgresql/postgresql.crt. This parameter is ignored if an SSL connection is not made.

sslkey

This parameter specifies the location for the secret key used for the client certificate. It can either specify a file name that will be used instead of the default ~/.postgresql/postgresql.key, or it can specify a key obtained from an external “engine” (engines are OpenSSL loadable modules). An external engine specification should consist of a colon-separated engine name and an engine-specific key identifier. This parameter is ignored if an SSL connection is not made.

sslrootcert

This parameter specifies the name of a file containing SSL certificate authority (CA) certificate(s). If the file exists, the server's certificate will be verified to be signed by one of these authorities. The default is ~/.postgresql/root.crt.

sslcrl

This parameter specifies the file name of the SSL certificate revocation list (CRL). Certificates listed in this file, if it exists, will be rejected while attempting to authenticate the server's certificate. The default is ~/.postgresql/root.crl.

requirepeer

krbsrvname

gsslib

GSS library to use for GSSAPI authentication. Only used on Windows. Set to gssapi to force libpq to use the GSSAPI library for authentication instead of the default SSPI.

service

target_session_attrs

If this parameter is set to read-write, only a connection in which read-write transactions are accepted by default is considered acceptable. The query SHOW transaction_read_only will be sent upon any successful connection; if it returns on, the connection will be closed. If multiple hosts were specified in the connection string, any remaining servers will be tried just as if the connection attempt had failed. The default value of this parameter, any, regards all connections as acceptable.

如果連線需要密碼（並且沒有指定密碼），使用者主目錄中的 .pgpass 檔案可以內含要使用的密碼。在 Microsoft Windows 上，該檔案路徑為 ％APPDATA％\postgresql\pgpass.conf（其中 ％APPDATA％ 指使用者組態檔案中的 Application Data 目錄）。或者，可以使用連接參數 passfile 或環境變數 PGPASSFILE 來指定密碼檔案。

該檔案應該包含以下格式的內容：

hostname:port:database:username:password

（您可以透過複製上面的內容並在 # 之前增加提醒註解到該檔案。）前四個字段中的每一個都可以是一個文字字串，或 *，它可以匹配任何內容。 將使用匹配目前連線參數第一行中的密碼字串。（因此，在使用通用配對字時首先輸入更具體的項目。）如果項目需要包含「:」或「\」，請使用「\」跳脫字元。localhost 的主機名稱匹配來自本地機器的 TCP（主機名localhost）和 Unix domain socket（pghost 為空或預設的 domain socket 路徑）連線。 在備援伺服器中，複製的資料庫名稱與主服務器上的串流備援連線匹配。資料庫字段的用處有限，因為使用者對同一叢集中的所有資料庫具有相同的密碼。

在 Unix 系統上，密碼檔案的權限必須禁止任何對所有其他人或組群的存取；透過 chmod 0600 ~/.pgpass 等命令來設定。如果權限不是如此嚴格設定，則該檔案將被忽略。在Microsoft Windows上，則假設檔案儲存在安全的目錄中，因此不會進行特殊的權限檢查。

以下環境變數可用於選擇預設連線參數值，如果呼叫變數沒有直接指定值， PQconnectdb，PQsetdbLogin 和 PQsetdb 將使用這些值。例如，這些有助於避免將資料庫連線訊息硬寫到簡單的用戶端應用程序中。

• PGHOST 的行為與 host 參數相同。

• PGHOSTADDR 的行為與 hostaddr 連線參數相同。這可以代替 PGHOST 或者除了 PGHOST 之外設定，以避免 DNS 查詢的開銷。

• PGPORT 的行為與 port 連線參數相同。

• PGDATABASE 的行為與 dbname 連線參數相同。

• PGUSER 的行為與 user 連線參數相同。

• PGPASSWORD 的行為與 password 連線參數相同。但由於安全因素，不建議使用此環境變數，因為某些作業系統允許非 root 使用者透過 ps 查看程序環境變數；而是考慮使用密碼檔案（參閱第 33.15 節）。

• PGPASSFILE 行為與 passfile 連線參數相同。

• PGSERVICE behaves the same as the service connection parameter.

• PGSERVICEFILE specifies the name of the per-user connection service file. If not set, it defaults to ~/.pg_service.conf (see Section 33.16).

• PGOPTIONS behaves the same as the options connection parameter.

• PGAPPNAME behaves the same as the application_name connection parameter.

• PGSSLMODE behaves the same as the sslmode connection parameter.

• PGREQUIRESSL behaves the same as the requiressl connection parameter. This environment variable is deprecated in favor of the PGSSLMODE variable; setting both variables suppresses the effect of this one.

• PGSSLCOMPRESSION behaves the same as the sslcompression connection parameter.

• PGSSLCERT behaves the same as the sslcert connection parameter.

• PGSSLKEY behaves the same as the sslkey connection parameter.

• PGSSLROOTCERT behaves the same as the sslrootcert connection parameter.

• PGSSLCRL behaves the same as the sslcrl connection parameter.

• PGREQUIREPEER behaves the same as the requirepeer connection parameter.

• PGKRBSRVNAME behaves the same as the krbsrvname connection parameter.

• PGGSSLIB behaves the same as the gsslib connection parameter.

• PGCONNECT_TIMEOUT behaves the same as the connect_timeout connection parameter.

• PGCLIENTENCODING behaves the same as the client_encoding connection parameter.

• PGTARGETSESSIONATTRS behaves the same as the target_session_attrs connection parameter.

以下環境變數可用於指定每個 PostgreSQL 連線的預設行為。（有關基於每個使用者或每個資料庫設定預設行為的方法，另請參閱 ALTER ROLE 和 ALTER DATABASE 指令。）

• PGDATESTYLE 設定日期/時間表示的預設樣式。 （相當於 SET datestyle TO ....）

• PGTZ設定預設時區。 （相當於 SET timezone TO ....）

• PGGEQO 設定 genetic 查詢最佳化程序的預設模式。（相當於 SET geqo TO ....）

有關這些環境變數的正確值的資訊，請參閱 SQL 指令 SET。

以下環境變數決定了 libpq 的內部行為；它們會覆寫編譯時的預設值。

• PGSYSCONFDIR 設定包含 pg_service.conf 檔案的目錄，在未來版本中可能用於設定其他系統範圍的組態檔案。

• PGLOCALEDIR 設定包含用於訊息本地化的區域設定檔案的目錄。

PostgreSQL has a large object facility, which provides stream-style access to user data that is stored in a special large-object structure. Streaming access is useful when working with data values that are too large to manipulate conveniently as a whole.

This chapter describes the implementation and the programming and query language interfaces to PostgreSQL large object data. We use the libpq C library for the examples in this chapter, but most programming interfaces native to PostgreSQL support equivalent functionality. Other interfaces might use the large object interface internally to provide generic support for large values. This is not described here.

These examples and others can be found in the directory src/test/examples in the source code distribution.

Example 33.1. libpq Example Program 1

Example 33.2. libpq Example Program 2

Example 33.3. libpq Example Program 3

This chapter describes the embedded SQL package for PostgreSQL. It was written by Linus Tolke (<linus@epact.se>) and Michael Meskes (<meskes@postgresql.org>). Originally it was written to work with C. It also works with C++, but it does not recognize all C++ constructs yet.

This documentation is quite incomplete. But since this interface is standardized, additional information can be found in many resources about SQL.

The information schema itself is a schema named information_schema. This schema automatically exists in all databases. The owner of this schema is the initial database user in the cluster, and that user naturally has all the privileges on this schema, including the ability to drop it (but the space savings achieved by that are minuscule).

By default, the information schema is not in the schema search path, so you need to access all objects in it through qualified names. Since the names of some of the objects in the information schema are generic names that might occur in user applications, you should be careful if you want to put the information schema in the path.

The columns of the information schema views use special data types that are defined in the information schema. These are defined as simple domains over ordinary built-in types. You should not use these types for work outside the information schema, but your applications must be prepared for them if they select from the information schema.

These types are:cardinal_number

A nonnegative integer.character_data

A character string (without specific maximum length).sql_identifier

A character string. This type is used for SQL identifiers, the type character_data is used for any other kind of text data.time_stamp

A domain over the type timestamp with time zoneyes_or_no

A character string domain that contains either YES or NO. This is used to represent Boolean (true/false) data in the information schema. (The information schema was invented before the typeboolean was added to the SQL standard, so this convention is necessary to keep the information schema backward compatible.)

Every column in the information schema has one of these five types.

The information schema consists of a set of views that contain information about the objects defined in the current database. The information schema is defined in the SQL standard and can therefore be expected to be portable and remain stable — unlike the system catalogs, which are specific to PostgreSQL and are modeled after implementation concerns. The information schema views do not, however, contain information about PostgreSQL-specific features; to inquire about those you need to query the system catalogs or other PostgreSQL-specific views.

Note

When querying the database for constraint information, it is possible for a standard-compliant query that expects to return one row to return several. This is because the SQL standard requires constraint names to be unique within a schema, but PostgreSQL does not enforce this restriction. PostgreSQL automatically-generated constraint names avoid duplicates in the same schema, but users can specify such duplicate names.

This problem can appear when querying information schema views such as check_constraint_routine_usage, check_constraints, domain_constraints, and referential_constraints. Some other views have similar issues but contain the table name to help distinguish duplicate rows, e.g., constraint_column_usage, constraint_table_usage, table_constraints.