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38.5. Basic Statements

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 38.5.2 and Section 38.5.3.

38.5.1. Assignment

An assignment of a value to a PL/pgSQL variable or row/record field is written as:

variable := expression;

As explained above, 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.

If the expression's result data type doesn't match the variable's data type, or the variable has a specific size/precision (like char(20)), the result value will be implicitly converted by the PL/pgSQL interpreter using the result type's output-function and the variable type's input-function. Note that this could potentially 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:

tax := subtotal * 0.06;
my_record.user_id := 20;

38.5.2. Executing a Command With No Result

For any SQL command that does not return rows, for example INSERT without a RETURNING clause, you can execute the command within a PL/pgSQL function just by writing the command.

Any PL/pgSQL variable name appearing in the command text is replaced by a parameter symbol, 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 38.10.1. As an example, if you write:

DECLARE
    key TEXT;
    delta INTEGER;
BEGIN
    ...
    UPDATE mytab SET val = val + delta WHERE id = key;

the command text seen by the main SQL engine will look like:

    UPDATE mytab SET val = val + $1 WHERE id = $2;

Although you don't normally have to think about this, it's helpful to know it when you need to make sense of syntax-error messages.

Caution

PL/pgSQL will substitute for any identifier matching one of the function's declared variables; it is not bright enough to know whether that's what you meant! Thus, it is a bad idea to use a variable name that is the same as any table, column, or function name that you need to reference in commands within the function. For more discussion see Section 38.10.1.

When executing a SQL command in this way, PL/pgSQL plans the command just once and re-uses the plan on subsequent executions, for the life of the database connection. The implications of this are discussed in detail in Section 38.10.2.

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:

PERFORM query;

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. PL/pgSQL variables will be substituted into the query just as for commands that return no result, 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 38.5.5).

Note: One might expect that writing SELECT directly would accomplish this result, but at present the only accepted way to do it is PERFORM. A 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:

PERFORM create_mv('cs_session_page_requests_mv', my_query);

38.5.3. Executing a Query with a Single-Row Result

The result of a 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,

SELECT select_expressions INTO [STRICT] target FROM ...;
INSERT ... RETURNING expressions INTO [STRICT] target;
UPDATE ... RETURNING expressions INTO [STRICT] target;
DELETE ... RETURNING expressions INTO [STRICT] target;

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 query, and the plan is cached, just as described above for commands that do not return rows. This works for SELECT, INSERT/UPDATE/DELETE with RETURNING, and utility commands that return row-set results (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 or a variable list is used as target, the query's result columns must exactly match the structure of the target as to number and data types, or a run-time error occurs. When a record variable is the target, it automatically configures itself to the row type of the query 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 query, or to nulls if the query 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 38.5.5) to determine whether a row was returned:

SELECT * INTO myrec FROM emp WHERE empname = myname;
IF NOT FOUND THEN
    RAISE EXCEPTION 'employee % not found', myname;
END IF;

If the STRICT option is specified, the query 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:

BEGIN
    SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
    EXCEPTION
        WHEN NO_DATA_FOUND THEN
            RAISE EXCEPTION 'employee % not found', myname;
        WHEN TOO_MANY_ROWS THEN
            RAISE EXCEPTION 'employee % not unique', myname;
END;

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.

Note: The STRICT option matches the behavior of Oracle PL/SQL's SELECT INTO and related statements.

To handle cases where you need to process multiple result rows from a SQL query, see Section 38.6.4.

38.5.4. 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 38.10.2) will not work in such scenarios. To handle this sort of problem, the EXECUTE statement is provided:

EXECUTE command-string [ INTO [STRICT] target ] [ USING expression [, ...] ];

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 prepared 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 a SQL command returning rows should be assigned. If a row or variable list is provided, it must exactly match the structure of the query's results (when a record variable is used, 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. If no rows are returned, NULL is assigned to the INTO variable(s). If no INTO clause is specified, the query results are discarded.

If the STRICT option is given, an error is reported unless the query 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:

EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
   INTO c
   USING checked_user, checked_date;

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:

EXECUTE 'SELECT count(*) FROM '
    || tabname::regclass
    || ' WHERE inserted_by = $1 AND inserted <= $2'
   INTO c
   USING checked_user, checked_date;

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 normally creates a generic plan and caches it for re-use. In situations where the best plan depends strongly on the parameter values, EXECUTE can be significantly faster; while when the plan is not sensitive to parameter values, re-planning will be a waste.

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 38-1. 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 38.11.1, which can save you some effort when translating said code to a more reasonable scheme.)

Dynamic values that are to be inserted into the constructed query require careful handling since they might themselves contain quote characters. An example (this assumes that you are using dollar quoting for the function as a whole, so the quote marks need not be doubled):

EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = '
        || quote_literal(newvalue)
        || ' WHERE key = '
        || quote_literal(keyvalue);

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 labelled 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,

EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = '
        || quote_nullable(newvalue)
        || ' WHERE key = '
        || quote_nullable(keyvalue);

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

     'WHERE key = ' || quote_nullable(keyvalue)

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

     'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)

(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:

EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = $$'
        || newvalue
        || '$$ WHERE key = '
        || quote_literal(keyvalue);

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.

A much larger example of a dynamic command and EXECUTE can be seen in Example 38-7, which builds and executes a CREATE FUNCTION command to define a new function.

38.5.5. 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:

GET DIAGNOSTICS variable = item [ , ... ];

This command allows retrieval of system status indicators. Each item is a key word identifying a state 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 ROW_COUNT, the number of rows processed by the last SQL command sent to the SQL engine, and RESULT_OID, the OID of the last row inserted by the most recent SQL command. Note that RESULT_OID is only useful after an INSERT command into a table containing OIDs.

An example:

GET DIAGNOSTICS integer_var = ROW_COUNT;

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 sets FOUND true if a row is assigned, false if no row is returned.

  • A PERFORM statement sets FOUND true if it produces (and discards) one or more rows, false if no row is produced.

  • UPDATE, INSERT, and DELETE statements set FOUND true if at least one row is affected, false if no row is affected.

  • A FETCH statement sets FOUND true if it returns a row, false if no row is returned.

  • A MOVE statement sets FOUND true if it successfully repositions the cursor, false otherwise.

  • A FOR statement sets FOUND true if it iterates one or more times, else false. This applies to all four variants of the FOR statement (integer FOR loops, record-set FOR loops, dynamic record-set FOR loops, and cursor FOR loops). FOUND is set this way when the FOR loop exits; inside the execution of the loop, FOUND is not modified by the FOR statement, although it might be changed by the execution of other statements within the loop body.

  • A RETURN QUERY and RETURN QUERY EXECUTE statements set FOUND true if the query returns at least one row, false if no row is returned.

FOUND is a local variable within each PL/pgSQL function; any changes to it affect only the current function.

38.5.6. 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:

NULL;

For example, the following two fragments of code are equivalent:

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            NULL;  -- ignore the error
    END;
    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN  -- ignore the error
    END;

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.

Comments


July 22, 2009, 6:23 p.m.

A work around on the limitation of NEW and OLD not being processed by EXECUTE is to create a DIM
variable and use it. See the example.

CREATE TABLE audit (
table_name CHAR( 32 ),
column_name CHAR( 32 ),
line_id INTEGER,
op_type SMALLINT,
op_date TIMESTAMP,
user_name CHAR( 32 ),
original_value VARCHAR( 255 ),
new_value VARCHAR( 255 )
);

COMMENT ON TABLE audit IS 'Generic audit table.';
COMMENT ON COLUMN audit.op_type IS 'Operation type: 1=INSERT; 2=UPDATE; 3=DELETE';

DECLARE
col information_schema.columns %ROWTYPE;
line_old TEXT;
columns_old[] CHAR( 255 );
i INTEGER;

BEGIN
line_old := TRIM( OLD::text );
columns_old := STRING_TO_ARRAY( linha_old, ',' );

-- Now you can use a variable column on an ordinary statement

i := 0;

FOR col IN SELECT * FROM information_schema.columns WHERE table_name = TG_RELNAME LOOP
i := i + 1;

INSERT INTO audit VALUES ( TG_RELNAME, col.column_name, OLD.oid, 1, NOW(), SESSION_USER, TRIM( columns_old[ i ] ), '' );

END LOOP;

END;

Obviously the above is a simplification of the process, and one will have to test for TG_OP as required by his/her own needs.

HTH

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