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dblink_connect — opens a persistent connection to a remote database
dblink_connect() establishes a connection to a remote PostgreSQL database. The server and database to be contacted are identified through a standard libpq connection string. Optionally, a name can be assigned to the connection. Multiple named connections can be open at once, but only one unnamed connection is permitted at a time. The connection will persist until closed or until the database session is ended.
The connection string may also be the name of an existing foreign server. It is recommended to use the foreign-data wrapper dblink_fdw when defining the foreign server. See the example below, as well as CREATE SERVER and CREATE USER MAPPING.
connname
The name to use for this connection; if omitted, an unnamed connection is opened, replacing any existing unnamed connection.connstr
libpq-style connection info string, for example hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd options=-csearch_path=. For details see Section 33.1.1. Alternatively, the name of a foreign server.
Returns status, which is always OK (since any error causes the function to throw an error instead of returning).
If untrusted users have access to a database that has not adopted a secure schema usage pattern, begin each session by removing publicly-writable schemas from search_path. One could, for example, add options=-csearch_path= to connstr. This consideration is not specific to dblink; it applies to every interface for executing arbitrary SQL commands.
Only superusers may use dblink_connect to create non-password-authenticated connections. If non-superusers need this capability, use dblink_connect_u instead.
It is unwise to choose connection names that contain equal signs, as this opens a risk of confusion with connection info strings in other dblink functions.
dblink_get_notify — retrieve async notifications on a connection
dblink_get_notify retrieves notifications on either the unnamed connection, or on a named connection if specified. To receive notifications via dblink, LISTEN must first be issued, using dblink_exec. For details see LISTEN and NOTIFY.
connname
The name of a named connection to get notifications on.
Returns setof (notify_name text, be_pid int, extra text), or an empty set if none.
dblink_build_sql_delete — builds a DELETE statement using supplied values for primary key field values
dblink_build_sql_delete can be useful in doing selective replication of a local table to a remote database. It builds a SQL DELETE command that will delete the row with the given primary key values.
relname
Name of a local relation, for example foo or myschema.mytab. Include double quotes if the name is mixed-case or contains special characters, for example "FooBar"; without quotes, the string will be folded to lower case.
primary_key_attnums
Attribute numbers (1-based) of the primary key fields, for example 1 2.
num_primary_key_atts
The number of primary key fields.
tgt_pk_att_vals_array
Values of the primary key fields to be used in the resulting DELETE command. Each field is represented in text form.
Returns the requested SQL statement as text.
As of PostgreSQL 9.0, the attribute numbers in primary_key_attnums are interpreted as logical column numbers, corresponding to the column's position in SELECT * FROM relname. Previous versions interpreted the numbers as physical column positions. There is a difference if any column(s) to the left of the indicated column have been dropped during the lifetime of the table.
dblink 模組可以讓你從資料庫連線中再連線到其他 PostgreSQL 資料庫。
另請參閱 postgres_fdw，它使用更現代且符合標準的基礎架構提供大致相同的功能。
dblink_exec — executes a command in a remote database
dblink_exec executes a command (that is, any SQL statement that doesn't return rows) in a remote database.
When two text arguments are given, the first one is first looked up as a persistent connection's name; if found, the command is executed on that connection. If not found, the first argument is treated as a connection info string as for dblink_connect, and the indicated connection is made just for the duration of this command.
connname
Name of the connection to use; omit this parameter to use the unnamed connection.
connstr
A connection info string, as previously described for dblink_connect.
sql
The SQL command that you wish to execute in the remote database, for example insert into foo values(0,'a','{"a0","b0","c0"}').
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function's return value is set to ERROR.
Returns status, either the command's status string or ERROR.
dblink_disconnect — closes a persistent connection to a remote database
dblink_disconnect() closes a connection previously opened by dblink_connect(). The form with no arguments closes an unnamed connection.
connname
The name of a named connection to be closed.
Returns status, which is always OK (since any error causes the function to throw an error instead of returning).
dblink_close — closes a cursor in a remote database
dblink_close closes a cursor previously opened with dblink_open.
connname
Name of the connection to use; omit this parameter to use the unnamed connection.
cursorname
The name of the cursor to close.
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function's return value is set to ERROR.
Returns status, either OK or ERROR.
If dblink_open started an explicit transaction block, and this is the last remaining open cursor in this connection, dblink_close will issue the matching COMMIT.
dblink_connect_u — opens a persistent connection to a remote database, insecurely
dblink_connect_u() is identical to dblink_connect(), except that it will allow non-superusers to connect using any authentication method.
If the remote server selects an authentication method that does not involve a password, then impersonation and subsequent escalation of privileges can occur, because the session will appear to have originated from the user as which the local PostgreSQL server runs. Also, even if the remote server does demand a password, it is possible for the password to be supplied from the server environment, such as a ~/.pgpass file belonging to the server's user. This opens not only a risk of impersonation, but the possibility of exposing a password to an untrustworthy remote server. Therefore, dblink_connect_u() is initially installed with all privileges revoked from PUBLIC, making it un-callable except by superusers. In some situations it may be appropriate to grant EXECUTE permission for dblink_connect_u() to specific users who are considered trustworthy, but this should be done with care. It is also recommended that any ~/.pgpass file belonging to the server's user not contain any records specifying a wildcard host name.
For further details see dblink_connect().
dblink_error_message — gets last error message on the named connection
dblink_error_message fetches the most recent remote error message for a given connection.
connname
Name of the connection to use.
Returns last error message, or OK if there has been no error in this connection.
When asynchronous queries are initiated by dblink_send_query, the error message associated with the connection might not get updated until the server's response message is consumed. This typically means that dblink_is_busy or dblink_get_result should be called prior to dblink_error_message, so that any error generated by the asynchronous query will be visible.
dblink_send_query — sends an async query to a remote database
dblink_send_query sends a query to be executed asynchronously, that is, without immediately waiting for the result. There must not be an async query already in progress on the connection.
After successfully dispatching an async query, completion status can be checked with dblink_is_busy, and the results are ultimately collected with dblink_get_result. It is also possible to attempt to cancel an active async query using dblink_cancel_query.
connname
Name of the connection to use.
sql
The SQL statement that you wish to execute in the remote database, for example select * from pg_class.
Returns 1 if the query was successfully dispatched, 0 otherwise.
dblink_open — opens a cursor in a remote database
dblink_open() opens a cursor in a remote database. The cursor can subsequently be manipulated with dblink_fetch() and dblink_close().
connname
Name of the connection to use; omit this parameter to use the unnamed connection.
cursorname
The name to assign to this cursor.
sql
The SELECT statement that you wish to execute in the remote database, for example select * from pg_class.
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function's return value is set to ERROR.
Returns status, either OK or ERROR.
Since a cursor can only persist within a transaction, dblink_open starts an explicit transaction block (BEGIN) on the remote side, if the remote side was not already within a transaction. This transaction will be closed again when the matching dblink_close is executed. Note that if you use dblink_exec to change data between dblink_open and dblink_close, and then an error occurs or you use dblink_disconnect before dblink_close, your change will be lost because the transaction will be aborted.
dblink_get_result — gets an async query result
dblink_get_result collects the results of an asynchronous query previously sent with dblink_send_query. If the query is not already completed, dblink_get_result will wait until it is.
connname
Name of the connection to use.
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function returns no rows.
For an async query (that is, a SQL statement returning rows), the function returns the row(s) produced by the query. To use this function, you will need to specify the expected set of columns, as previously discussed for dblink.
For an async command (that is, a SQL statement not returning rows), the function returns a single row with a single text column containing the command's status string. It is still necessary to specify that the result will have a single text column in the calling FROM clause.
This function must be called if dblink_send_query returned 1. It must be called once for each query sent, and one additional time to obtain an empty set result, before the connection can be used again.
When using dblink_send_query and dblink_get_result, dblink fetches the entire remote query result before returning any of it to the local query processor. If the query returns a large number of rows, this can result in transient memory bloat in the local session. It may be better to open such a query as a cursor with dblink_open and then fetch a manageable number of rows at a time. Alternatively, use plain dblink(), which avoids memory bloat by spooling large result sets to disk.
dblink_cancel_query — cancels any active query on the named connection
dblink_cancel_query attempts to cancel any query that is in progress on the named connection. Note that this is not certain to succeed (since, for example, the remote query might already have finished). A cancel request simply improves the odds that the query will fail soon. You must still complete the normal query protocol, for example by calling dblink_get_result.
connname
Name of the connection to use.
Returns OK if the cancel request has been sent, or the text of an error message on failure.
dblink — executes a query in a remote database
dblink executes a query (usually a SELECT, but it can be any SQL statement that returns rows) in a remote database.
When two text arguments are given, the first one is first looked up as a persistent connection's name; if found, the command is executed on that connection. If not found, the first argument is treated as a connection info string as for dblink_connect, and the indicated connection is made just for the duration of this command.
connname
Name of the connection to use; omit this parameter to use the unnamed connection.connstr
A connection info string, as previously described for dblink_connect.
sql
The SQL query that you wish to execute in the remote database, for example select * from foo.
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function returns no rows.
The function returns the row(s) produced by the query. Since dblink can be used with any query, it is declared to return record, rather than specifying any particular set of columns. This means that you must specify the expected set of columns in the calling query — otherwise PostgreSQL would not know what to expect. Here is an example:
The “alias” part of the FROM clause must specify the column names and types that the function will return. (Specifying column names in an alias is actually standard SQL syntax, but specifying column types is a PostgreSQL extension.) This allows the system to understand what * should expand to, and what proname in the WHERE clause refers to, in advance of trying to execute the function. At run time, an error will be thrown if the actual query result from the remote database does not have the same number of columns shown in the FROM clause. The column names need not match, however, and dblink does not insist on exact type matches either. It will succeed so long as the returned data strings are valid input for the column type declared in the FROM clause.
A convenient way to use dblink with predetermined queries is to create a view. This allows the column type information to be buried in the view, instead of having to spell it out in every query. For example,
dblink_build_sql_insert — builds an INSERT statement using a local tuple, replacing the primary key field values with alternative supplied values
dblink_build_sql_insert can be useful in doing selective replication of a local table to a remote database. It selects a row from the local table based on primary key, and then builds a SQL INSERT command that will duplicate that row, but with the primary key values replaced by the values in the last argument. (To make an exact copy of the row, just specify the same values for the last two arguments.)
relname
Name of a local relation, for example foo or myschema.mytab. Include double quotes if the name is mixed-case or contains special characters, for example "FooBar"; without quotes, the string will be folded to lower case.
primary_key_attnums
Attribute numbers (1-based) of the primary key fields, for example 1 2.
num_primary_key_atts
The number of primary key fields.
src_pk_att_vals_array
Values of the primary key fields to be used to look up the local tuple. Each field is represented in text form. An error is thrown if there is no local row with these primary key values.
tgt_pk_att_vals_array
Values of the primary key fields to be placed in the resulting INSERT command. Each field is represented in text form.
Returns the requested SQL statement as text.
As of PostgreSQL 9.0, the attribute numbers in primary_key_attnums are interpreted as logical column numbers, corresponding to the column's position in SELECT * FROM relname. Previous versions interpreted the numbers as physical column positions. There is a difference if any column(s) to the left of the indicated column have been dropped during the lifetime of the table.
dblink_fetch — returns rows from an open cursor in a remote database
dblink_fetch fetches rows from a cursor previously established by dblink_open.
connname
Name of the connection to use; omit this parameter to use the unnamed connection.
cursorname
The name of the cursor to fetch from.
howmany
The maximum number of rows to retrieve. The next howmany rows are fetched, starting at the current cursor position, moving forward. Once the cursor has reached its end, no more rows are produced.
fail_on_error
If true (the default when omitted) then an error thrown on the remote side of the connection causes an error to also be thrown locally. If false, the remote error is locally reported as a NOTICE, and the function returns no rows.
The function returns the row(s) fetched from the cursor. To use this function, you will need to specify the expected set of columns, as previously discussed for dblink.
On a mismatch between the number of return columns specified in the FROM clause, and the actual number of columns returned by the remote cursor, an error will be thrown. In this event, the remote cursor is still advanced by as many rows as it would have been if the error had not occurred. The same is true for any other error occurring in the local query after the remote FETCH has been done.
dblink_build_sql_update — builds an UPDATE statement using a local tuple, replacing the primary key field values with alternative supplied values
dblink_build_sql_update can be useful in doing selective replication of a local table to a remote database. It selects a row from the local table based on primary key, and then builds a SQL UPDATE command that will duplicate that row, but with the primary key values replaced by the values in the last argument. (To make an exact copy of the row, just specify the same values for the last two arguments.) The UPDATE command always assigns all fields of the row — the main difference between this and dblink_build_sql_insert is that it's assumed that the target row already exists in the remote table.
relname
Name of a local relation, for example foo or myschema.mytab. Include double quotes if the name is mixed-case or contains special characters, for example "FooBar"; without quotes, the string will be folded to lower case.
primary_key_attnums
Attribute numbers (1-based) of the primary key fields, for example 1 2.num_primary_key_atts
The number of primary key fields.
src_pk_att_vals_array
Values of the primary key fields to be used to look up the local tuple. Each field is represented in text form. An error is thrown if there is no local row with these primary key values.
tgt_pk_att_vals_array
Values of the primary key fields to be placed in the resulting UPDATE command. Each field is represented in text form.
Returns the requested SQL statement as text.
As of PostgreSQL 9.0, the attribute numbers in primary_key_attnums are interpreted as logical column numbers, corresponding to the column's position in SELECT * FROM relname. Previous versions interpreted the numbers as physical column positions. There is a difference if any column(s) to the left of the indicated column have been dropped during the lifetime of the table.
dblink_get_pkey — returns the positions and field names of a relation's primary key fields
dblink_get_pkey provides information about the primary key of a relation in the local database. This is sometimes useful in generating queries to be sent to remote databases.
relname
Name of a local relation, for example foo or myschema.mytab. Include double quotes if the name is mixed-case or contains special characters, for example "FooBar"; without quotes, the string will be folded to lower case.
Returns one row for each primary key field, or no rows if the relation has no primary key. The result row type is defined as
The position column simply runs from 1 to N; it is the number of the field within the primary key, not the number within the table's columns.