43.5. 基本語法
In this section and the following ones, we describe all the statement types that are explicitly understood by PL/pgSQL. Anything not recognized as one of these statement types is presumed to be an SQL command and is sent to the main database engine to execute, as described in Section 43.5.2.
43.5.1. Assignment
An assignment of a value to a PL/pgSQL variable is written as:
As explained previously, the expression in such a statement is evaluated by means of an SQL SELECT
command sent to the main database engine. The expression must yield a single value (possibly a row value, if the variable is a row or record variable). The target variable can be a simple variable (optionally qualified with a block name), a field of a row or record target, or an element or slice of an array target. Equal (=
) can be used instead of PL/SQL-compliant :=
.
If the expression's result data type doesn't match the variable's data type, the value will be coerced as though by an assignment cast (see Section 10.4). If no assignment cast is known for the pair of data types involved, the PL/pgSQL interpreter will attempt to convert the result value textually, that is by applying the result type's output function followed by the variable type's input function. Note that this could result in run-time errors generated by the input function, if the string form of the result value is not acceptable to the input function.
Examples:
43.5.2. Executing SQL Commands
In general, any SQL command that does not return rows can be executed within a PL/pgSQL function just by writing the command. For example, you could create and fill a table by writing
If the command does return rows (for example SELECT
, or INSERT
/UPDATE
/DELETE
with RETURNING
), there are two ways to proceed. When the command will return at most one row, or you only care about the first row of output, write the command as usual but add an INTO
clause to capture the output, as described in Section 43.5.3. To process all of the output rows, write the command as the data source for a FOR
loop, as described in Section 43.6.6.
Usually it is not sufficient just to execute statically-defined SQL commands. Typically you'll want a command to use varying data values, or even to vary in more fundamental ways such as by using different table names at different times. Again, there are two ways to proceed depending on the situation.
PL/pgSQL variable values can be automatically inserted into optimizable SQL commands, which are SELECT
, INSERT
, UPDATE
, DELETE
, and certain utility commands that incorporate one of these, such as EXPLAIN
and CREATE TABLE ... AS SELECT
. In these commands, any PL/pgSQL variable name appearing in the command text is replaced by a query parameter, and then the current value of the variable is provided as the parameter value at run time. This is exactly like the processing described earlier for expressions; for details see Section 43.11.1.
When executing an optimizable SQL command in this way, PL/pgSQL may cache and re-use the execution plan for the command, as discussed in Section 43.11.2.
Non-optimizable SQL commands (also called utility commands) are not capable of accepting query parameters. So automatic substitution of PL/pgSQL variables does not work in such commands. To include non-constant text in a utility command executed from PL/pgSQL, you must build the utility command as a string and then EXECUTE
it, as discussed in Section 43.5.4.
EXECUTE
must also be used if you want to modify the command in some other way than supplying a data value, for example by changing a table name.
Sometimes it is useful to evaluate an expression or SELECT
query but discard the result, for example when calling a function that has side-effects but no useful result value. To do this in PL/pgSQL, use the PERFORM
statement:
This executes query
and discards the result. Write the query
the same way you would write an SQL SELECT
command, but replace the initial keyword SELECT
with PERFORM
. For WITH
queries, use PERFORM
and then place the query in parentheses. (In this case, the query can only return one row.) PL/pgSQL variables will be substituted into the query just as described above, and the plan is cached in the same way. Also, the special variable FOUND
is set to true if the query produced at least one row, or false if it produced no rows (see Section 43.5.5).
Note
One might expect that writing SELECT
directly would accomplish this result, but at present the only accepted way to do it is PERFORM
. An SQL command that can return rows, such as SELECT
, will be rejected as an error unless it has an INTO
clause as discussed in the next section.
An example:
43.5.3. Executing a Command with a Single-Row Result
The result of an SQL command yielding a single row (possibly of multiple columns) can be assigned to a record variable, row-type variable, or list of scalar variables. This is done by writing the base SQL command and adding an INTO
clause. For example,
where target
can be a record variable, a row variable, or a comma-separated list of simple variables and record/row fields. PL/pgSQL variables will be substituted into the rest of the command (that is, everything but the INTO
clause) just as described above, and the plan is cached in the same way. This works for SELECT
, INSERT
/UPDATE
/DELETE
with RETURNING
, and certain utility commands that return row sets, such as EXPLAIN
. Except for the INTO
clause, the SQL command is the same as it would be written outside PL/pgSQL.
Tip
Note that this interpretation of SELECT
with INTO
is quite different from PostgreSQL's regular SELECT INTO
command, wherein the INTO
target is a newly created table. If you want to create a table from a SELECT
result inside a PL/pgSQL function, use the syntax CREATE TABLE ... AS SELECT
.
If a row variable or a variable list is used as target, the command's result columns must exactly match the structure of the target as to number and data types, or else a run-time error occurs. When a record variable is the target, it automatically configures itself to the row type of the command's result columns.
The INTO
clause can appear almost anywhere in the SQL command. Customarily it is written either just before or just after the list of select_expressions
in a SELECT
command, or at the end of the command for other command types. It is recommended that you follow this convention in case the PL/pgSQL parser becomes stricter in future versions.
If STRICT
is not specified in the INTO
clause, then target
will be set to the first row returned by the command, or to nulls if the command returned no rows. (Note that “the first row” is not well-defined unless you've used ORDER BY
.) Any result rows after the first row are discarded. You can check the special FOUND
variable (see Section 43.5.5) to determine whether a row was returned:
If the STRICT
option is specified, the command must return exactly one row or a run-time error will be reported, either NO_DATA_FOUND
(no rows) or TOO_MANY_ROWS
(more than one row). You can use an exception block if you wish to catch the error, for example:
Successful execution of a command with STRICT
always sets FOUND
to true.
For INSERT
/UPDATE
/DELETE
with RETURNING
, PL/pgSQL reports an error for more than one returned row, even when STRICT
is not specified. This is because there is no option such as ORDER BY
with which to determine which affected row should be returned.
If print_strict_params
is enabled for the function, then when an error is thrown because the requirements of STRICT
are not met, the DETAIL
part of the error message will include information about the parameters passed to the command. You can change the print_strict_params
setting for all functions by setting plpgsql.print_strict_params
, though only subsequent function compilations will be affected. You can also enable it on a per-function basis by using a compiler option, for example:
On failure, this function might produce an error message such as
Note
The STRICT
option matches the behavior of Oracle PL/SQL's SELECT INTO
and related statements.
43.5.4. Executing Dynamic Commands
Oftentimes you will want to generate dynamic commands inside your PL/pgSQL functions, that is, commands that will involve different tables or different data types each time they are executed. PL/pgSQL's normal attempts to cache plans for commands (as discussed in Section 43.11.2) will not work in such scenarios. To handle this sort of problem, the EXECUTE
statement is provided:
where command-string
is an expression yielding a string (of type text
) containing the command to be executed. The optional target
is a record variable, a row variable, or a comma-separated list of simple variables and record/row fields, into which the results of the command will be stored. The optional USING
expressions supply values to be inserted into the command.
No substitution of PL/pgSQL variables is done on the computed command string. Any required variable values must be inserted in the command string as it is constructed; or you can use parameters as described below.
Also, there is no plan caching for commands executed via EXECUTE
. Instead, the command is always planned each time the statement is run. Thus the command string can be dynamically created within the function to perform actions on different tables and columns.
The INTO
clause specifies where the results of an SQL command returning rows should be assigned. If a row variable or variable list is provided, it must exactly match the structure of the command's results; if a record variable is provided, it will configure itself to match the result structure automatically. If multiple rows are returned, only the first will be assigned to the INTO
variable(s). If no rows are returned, NULL is assigned to the INTO
variable(s). If no INTO
clause is specified, the command results are discarded.
If the STRICT
option is given, an error is reported unless the command produces exactly one row.
The command string can use parameter values, which are referenced in the command as $1
, $2
, etc. These symbols refer to values supplied in the USING
clause. This method is often preferable to inserting data values into the command string as text: it avoids run-time overhead of converting the values to text and back, and it is much less prone to SQL-injection attacks since there is no need for quoting or escaping. An example is:
Note that parameter symbols can only be used for data values — if you want to use dynamically determined table or column names, you must insert them into the command string textually. For example, if the preceding query needed to be done against a dynamically selected table, you could do this:
A cleaner approach is to use format()
's %I
specification to insert table or column names with automatic quoting:
(This example relies on the SQL rule that string literals separated by a newline are implicitly concatenated.)
Another restriction on parameter symbols is that they only work in optimizable SQL commands (SELECT
, INSERT
, UPDATE
, DELETE
, MERGE
, and certain commands containing one of these). In other statement types (generically called utility statements), you must insert values textually even if they are just data values.
An EXECUTE
with a simple constant command string and some USING
parameters, as in the first example above, is functionally equivalent to just writing the command directly in PL/pgSQL and allowing replacement of PL/pgSQL variables to happen automatically. The important difference is that EXECUTE
will re-plan the command on each execution, generating a plan that is specific to the current parameter values; whereas PL/pgSQL may otherwise create a generic plan and cache it for re-use. In situations where the best plan depends strongly on the parameter values, it can be helpful to use EXECUTE
to positively ensure that a generic plan is not selected.
SELECT INTO
is not currently supported within EXECUTE
; instead, execute a plain SELECT
command and specify INTO
as part of the EXECUTE
itself.
Note
The PL/pgSQL EXECUTE
statement is not related to the EXECUTE
SQL statement supported by the PostgreSQL server. The server's EXECUTE
statement cannot be used directly within PL/pgSQL functions (and is not needed).
Example 43.1. Quoting Values in Dynamic Queries
When working with dynamic commands you will often have to handle escaping of single quotes. The recommended method for quoting fixed text in your function body is dollar quoting. (If you have legacy code that does not use dollar quoting, please refer to the overview in Section 43.12.1, which can save you some effort when translating said code to a more reasonable scheme.)
Dynamic values require careful handling since they might contain quote characters. An example using format()
(this assumes that you are dollar quoting the function body so quote marks need not be doubled):
It is also possible to call the quoting functions directly:
This example demonstrates the use of the quote_ident
and quote_literal
functions (see Section 9.4). For safety, expressions containing column or table identifiers should be passed through quote_ident
before insertion in a dynamic query. Expressions containing values that should be literal strings in the constructed command should be passed through quote_literal
. These functions take the appropriate steps to return the input text enclosed in double or single quotes respectively, with any embedded special characters properly escaped.
Because quote_literal
is labeled STRICT
, it will always return null when called with a null argument. In the above example, if newvalue
or keyvalue
were null, the entire dynamic query string would become null, leading to an error from EXECUTE
. You can avoid this problem by using the quote_nullable
function, which works the same as quote_literal
except that when called with a null argument it returns the string NULL
. For example,
If you are dealing with values that might be null, you should usually use quote_nullable
in place of quote_literal
.
As always, care must be taken to ensure that null values in a query do not deliver unintended results. For example the WHERE
clause
will never succeed if keyvalue
is null, because the result of using the equality operator =
with a null operand is always null. If you wish null to work like an ordinary key value, you would need to rewrite the above as
(At present, IS NOT DISTINCT FROM
is handled much less efficiently than =
, so don't do this unless you must. See Section 9.2 for more information on nulls and IS DISTINCT
.)
Note that dollar quoting is only useful for quoting fixed text. It would be a very bad idea to try to write this example as:
because it would break if the contents of newvalue
happened to contain $$
. The same objection would apply to any other dollar-quoting delimiter you might pick. So, to safely quote text that is not known in advance, you must use quote_literal
, quote_nullable
, or quote_ident
, as appropriate.
Dynamic SQL statements can also be safely constructed using the format
function (see Section 9.4.1). For example:
%I
is equivalent to quote_ident
, and %L
is equivalent to quote_nullable
. The format
function can be used in conjunction with the USING
clause:
This form is better because the variables are handled in their native data type format, rather than unconditionally converting them to text and quoting them via %L
. It is also more efficient.
A much larger example of a dynamic command and EXECUTE
can be seen in Example 43.10, which builds and executes a CREATE FUNCTION
command to define a new function.
43.5.5. Obtaining the Result Status
There are several ways to determine the effect of a command. The first method is to use the GET DIAGNOSTICS
command, which has the form:
This command allows retrieval of system status indicators. CURRENT
is a noise word (but see also GET STACKED DIAGNOSTICS
in Section 43.6.8.1). Each item
is a key word identifying a status value to be assigned to the specified variable
(which should be of the right data type to receive it). The currently available status items are shown in Table 43.1. Colon-equal (:=
) can be used instead of the SQL-standard =
token. An example:
Table 43.1. Available Diagnostics Items
The second method to determine the effects of a command is to check the special variable named FOUND
, which is of type boolean
. FOUND
starts out false within each PL/pgSQL function call. It is set by each of the following types of statements:
A
SELECT INTO
statement setsFOUND
true if a row is assigned, false if no row is returned.A
PERFORM
statement setsFOUND
true if it produces (and discards) one or more rows, false if no row is produced.UPDATE
,INSERT
,DELETE
, andMERGE
statements setFOUND
true if at least one row is affected, false if no row is affected.A
FETCH
statement setsFOUND
true if it returns a row, false if no row is returned.A
MOVE
statement setsFOUND
true if it successfully repositions the cursor, false otherwise.A
FOR
orFOREACH
statement setsFOUND
true if it iterates one or more times, else false.FOUND
is set this way when the loop exits; inside the execution of the loop,FOUND
is not modified by the loop statement, although it might be changed by the execution of other statements within the loop body.RETURN QUERY
andRETURN QUERY EXECUTE
statements setFOUND
true if the query returns at least one row, false if no row is returned.
Other PL/pgSQL statements do not change the state of FOUND
. Note in particular that EXECUTE
changes the output of GET DIAGNOSTICS
, but does not change FOUND
.
FOUND
is a local variable within each PL/pgSQL function; any changes to it affect only the current function.
43.5.6. Doing Nothing At All
Sometimes a placeholder statement that does nothing is useful. For example, it can indicate that one arm of an if/then/else chain is deliberately empty. For this purpose, use the NULL
statement:
For example, the following two fragments of code are equivalent:
Which is preferable is a matter of taste.
Note
In Oracle's PL/SQL, empty statement lists are not allowed, and so NULL
statements are required for situations such as this. PL/pgSQL allows you to just write nothing, instead.