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37.7. Control Structures

Control structures are probably the most useful (and important) part of PL/pgSQL. With PL/pgSQL's control structures, you can manipulate PostgreSQL data in a very flexible and powerful way.

37.7.1. Returning From a Function

There are two commands available that allow you to return data from a function: RETURN and RETURN NEXT.

37.7.1.1. RETURN

RETURN expression;

RETURN with an expression terminates the function and returns the value of expression to the caller. This form is to be used for PL/pgSQL functions that do not return a set.

When returning a scalar type, any expression can be used. The expression's result will be automatically cast into the function's return type as described for assignments. To return a composite (row) value, you must write a record or row variable as the expression.

If you declared the function with output parameters, write just RETURN with no expression. The current values of the output parameter variables will be returned.

If you declared the function to return void, a RETURN statement can be used to exit the function early; but do not write an expression following RETURN.

The return value of a function cannot be left undefined. If control reaches the end of the top-level block of the function without hitting a RETURN statement, a run-time error will occur. This restriction does not apply to functions with output parameters and functions returning void, however. In those cases a RETURN statement is automatically executed if the top-level block finishes.

37.7.1.2. RETURN NEXT

RETURN NEXT expression;

When a PL/pgSQL function is declared to return SETOF sometype, the procedure to follow is slightly different. In that case, the individual items to return are specified in RETURN NEXT commands, and then a final RETURN command with no argument is used to indicate that the function has finished executing. RETURN NEXT can be used with both scalar and composite data types; with a composite result type, an entire "table" of results will be returned.

RETURN NEXT does not actually return from the function — it simply saves away the value of the expression. Execution then continues with the next statement in the PL/pgSQL function. As successive RETURN NEXT commands are executed, the result set is built up. A final RETURN, which should have no argument, causes control to exit the function (or you can just let control reach the end of the function).

If you declared the function with output parameters, write just RETURN NEXT with no expression. The current values of the output parameter variable(s) will be saved for eventual return. Note that you must declare the function as returning SETOF record when there are multiple output parameters, or SETOF sometype when there is just one output parameter of type sometype, in order to create a set-returning function with output parameters.

Functions that use RETURN NEXT should be called in the following fashion:

SELECT * FROM some_func();

That is, the function must be used as a table source in a FROM clause.

Note: The current implementation of RETURN NEXT for PL/pgSQL stores the entire result set before returning from the function, as discussed above. That means that if a PL/pgSQL function produces a very large result set, performance may be poor: data will be written to disk to avoid memory exhaustion, but the function itself will not return until the entire result set has been generated. A future version of PL/pgSQL may allow users to define set-returning functions that do not have this limitation. Currently, the point at which data begins being written to disk is controlled by the work_mem configuration variable. Administrators who have sufficient memory to store larger result sets in memory should consider increasing this parameter.

37.7.2. Conditionals

IF statements let you execute commands based on certain conditions. PL/pgSQL has five forms of IF:

  • IF ... THEN

  • IF ... THEN ... ELSE

  • IF ... THEN ... ELSE IF

  • IF ... THEN ... ELSIF ... THEN ... ELSE

  • IF ... THEN ... ELSEIF ... THEN ... ELSE

37.7.2.1. IF-THEN

IF boolean-expression THEN
    statements
END IF;

IF-THEN statements are the simplest form of IF. The statements between THEN and END IF will be executed if the condition is true. Otherwise, they are skipped.

Example:

IF v_user_id <> 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;

37.7.2.2. IF-THEN-ELSE

IF boolean-expression THEN
    statements
ELSE
    statements
END IF;

IF-THEN-ELSE statements add to IF-THEN by letting you specify an alternative set of statements that should be executed if the condition evaluates to false.

Examples:

IF parentid IS NULL OR parentid = ''
THEN
    RETURN fullname;
ELSE
    RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
IF v_count > 0 THEN 
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;

37.7.2.3. IF-THEN-ELSE IF

IF statements can be nested, as in the following example:

IF demo_row.sex = 'm' THEN
    pretty_sex := 'man';
ELSE
    IF demo_row.sex = 'f' THEN
        pretty_sex := 'woman';
    END IF;
END IF;

When you use this form, you are actually nesting an IF statement inside the ELSE part of an outer IF statement. Thus you need one END IF statement for each nested IF and one for the parent IF-ELSE. This is workable but grows tedious when there are many alternatives to be checked. Hence the next form.

37.7.2.4. IF-THEN-ELSIF-ELSE

IF boolean-expression THEN
    statements
[ ELSIF boolean-expression THEN
    statements
[ ELSIF boolean-expression THEN
    statements
    ...]]
[ ELSE
    statements ]
END IF;

IF-THEN-ELSIF-ELSE provides a more convenient method of checking many alternatives in one statement. Formally it is equivalent to nested IF-THEN-ELSE-IF-THEN commands, but only one END IF is needed.

Here is an example:

IF number = 0 THEN
    result := 'zero';
ELSIF number > 0 THEN 
    result := 'positive';
ELSIF number < 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;

37.7.2.5. IF-THEN-ELSEIF-ELSE

ELSEIF is an alias for ELSIF.

37.7.3. Simple Loops

With the LOOP, EXIT, CONTINUE, WHILE, and FOR statements, you can arrange for your PL/pgSQL function to repeat a series of commands.

37.7.3.1. LOOP

[ <<label>> ]
LOOP
    statements
END LOOP [ label ];

LOOP defines an unconditional loop that is repeated indefinitely until terminated by an EXIT or RETURN statement. The optional label can be used by EXIT and CONTINUE statements in nested loops to specify which loop the statement should be applied to.

37.7.3.2. EXIT

EXIT [ label ] [ WHEN expression ];

If no label is given, the innermost loop is terminated and the statement following END LOOP is executed next. If label is given, it must be the label of the current or some outer level of nested loop or block. Then the named loop or block is terminated and control continues with the statement after the loop's/block's corresponding END.

If WHEN is specified, the loop exit occurs only if expression is true. Otherwise, control passes to the statement after EXIT.

EXIT can be used with all types of loops; it is not limited to use with unconditional loops. When used with a BEGIN block, EXIT passes control to the next statement after the end of the block.

Examples:

LOOP
    -- some computations
    IF count > 0 THEN
        EXIT;  -- exit loop
    END IF;
END LOOP;

LOOP
    -- some computations
    EXIT WHEN count > 0;  -- same result as previous example
END LOOP;

BEGIN
    -- some computations
    IF stocks > 100000 THEN
        EXIT;  -- causes exit from the BEGIN block
    END IF;
END;

37.7.3.3. CONTINUE

CONTINUE [ label ] [ WHEN expression ];

If no label is given, the next iteration of the innermost loop is begun. That is, control is passed back to the loop control expression (if any), and the body of the loop is re-evaluated. If label is present, it specifies the label of the loop whose execution will be continued.

If WHEN is specified, the next iteration of the loop is begun only if expression is true. Otherwise, control passes to the statement after CONTINUE.

CONTINUE can be used with all types of loops; it is not limited to use with unconditional loops.

Examples:

LOOP
    -- some computations
    EXIT WHEN count > 100;
    CONTINUE WHEN count < 50;
    -- some computations for count IN [50 .. 100] 
END LOOP;

37.7.3.4. WHILE

[ <<label>> ]
WHILE expression LOOP
    statements
END LOOP [ label ];

The WHILE statement repeats a sequence of statements so long as the condition expression evaluates to true. The condition is checked just before each entry to the loop body.

For example:

WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
    -- some computations here
END LOOP;

WHILE NOT boolean_expression LOOP
    -- some computations here
END LOOP;

37.7.3.5. FOR (integer variant)

[ <<label>> ]
FOR name IN [ REVERSE ] expression .. expression [ BY expression ] LOOP
    statements
END LOOP [ label ];

This form of FOR creates a loop that iterates over a range of integer values. The variable name is automatically defined as type integer and exists only inside the loop (any existing definition of the variable name is ignored within the loop). The two expressions giving the lower and upper bound of the range are evaluated once when entering the loop. If the BY clause isn't specified the iteration step is 1 otherwise it's the value specified in the BY clause. If REVERSE is specified then the step value is considered negative.

Some examples of integer FOR loops:

FOR i IN 1..10 LOOP
    -- some computations here
    RAISE NOTICE 'i is %', i;
END LOOP;

FOR i IN REVERSE 10..1 LOOP
    -- some computations here
END LOOP;

FOR i IN REVERSE 10..1 BY 2 LOOP
    -- some computations here
    RAISE NOTICE 'i is %', i;
END LOOP;

If the lower bound is greater than the upper bound (or less than, in the REVERSE case), the loop body is not executed at all. No error is raised.

37.7.4. Looping Through Query Results

Using a different type of FOR loop, you can iterate through the results of a query and manipulate that data accordingly. The syntax is:

[ <<label>> ]
FOR target IN query LOOP
    statements
END LOOP [ label ];

The target is a record variable, row variable, or comma-separated list of scalar variables. The target is successively assigned each row resulting from the query and the loop body is executed for each row. Here is an example:

CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS $$
DECLARE
    mviews RECORD;
BEGIN
    PERFORM cs_log('Refreshing materialized views...');

    FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP

        -- Now "mviews" has one record from cs_materialized_views

        PERFORM cs_log('Refreshing materialized view ' || quote_ident(mviews.mv_name) || ' ...');
        EXECUTE 'TRUNCATE TABLE ' || quote_ident(mviews.mv_name);
        EXECUTE 'INSERT INTO ' || quote_ident(mviews.mv_name) || ' ' || mviews.mv_query;
    END LOOP;

    PERFORM cs_log('Done refreshing materialized views.');
    RETURN 1;
END;
$$ LANGUAGE plpgsql;

If the loop is terminated by an EXIT statement, the last assigned row value is still accessible after the loop.

The query used in this type of FOR statement can be any SQL command that returns rows to the caller: SELECT is the most common case, but you can also use INSERT, UPDATE, or DELETE with a RETURNING clause. Some utility commands such as EXPLAIN will work too.

The FOR-IN-EXECUTE statement is another way to iterate over rows:

[ <<label>> ]
FOR target IN EXECUTE text_expression LOOP 
    statements
END LOOP [ label ];

This is like the previous form, except that the source query is specified as a string expression, which is evaluated and replanned on each entry to the FOR loop. This allows the programmer to choose the speed of a preplanned query or the flexibility of a dynamic query, just as with a plain EXECUTE statement.

Note: The PL/pgSQL parser presently distinguishes the two kinds of FOR loops (integer or query result) by checking whether .. appears outside any parentheses between IN and LOOP. If .. is not seen then the loop is presumed to be a loop over rows. Mistyping the .. is thus likely to lead to a complaint along the lines of "loop variable of loop over rows must be a record or row variable or list of scalar variables", rather than the simple syntax error one might expect to get.

37.7.5. Trapping Errors

By default, any error occurring in a PL/pgSQL function aborts execution of the function, and indeed of the surrounding transaction as well. You can trap errors and recover from them by using a BEGIN block with an EXCEPTION clause. The syntax is an extension of the normal syntax for a BEGIN block:

[ <<label>> ]
[ DECLARE
    declarations ]
BEGIN
    statements
EXCEPTION
    WHEN condition [ OR condition ... ] THEN
        handler_statements
    [ WHEN condition [ OR condition ... ] THEN
          handler_statements
      ... ]
END;

If no error occurs, this form of block simply executes all the statements, and then control passes to the next statement after END. But if an error occurs within the statements, further processing of the statements is abandoned, and control passes to the EXCEPTION list. The list is searched for the first condition matching the error that occurred. If a match is found, the corresponding handler_statements are executed, and then control passes to the next statement after END. If no match is found, the error propagates out as though the EXCEPTION clause were not there at all: the error can be caught by an enclosing block with EXCEPTION, or if there is none it aborts processing of the function.

The condition names can be any of those shown in Appendix A. A category name matches any error within its category. The special condition name OTHERS matches every error type except QUERY_CANCELED. (It is possible, but often unwise, to trap QUERY_CANCELED by name.) Condition names are not case-sensitive.

If a new error occurs within the selected handler_statements, it cannot be caught by this EXCEPTION clause, but is propagated out. A surrounding EXCEPTION clause could catch it.

When an error is caught by an EXCEPTION clause, the local variables of the PL/pgSQL function remain as they were when the error occurred, but all changes to persistent database state within the block are rolled back. As an example, consider this fragment:

    INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
    BEGIN
        UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
        x := x + 1;
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            RAISE NOTICE 'caught division_by_zero';
            RETURN x;
    END;

When control reaches the assignment to y, it will fail with a division_by_zero error. This will be caught by the EXCEPTION clause. The value returned in the RETURN statement will be the incremented value of x, but the effects of the UPDATE command will have been rolled back. The INSERT command preceding the block is not rolled back, however, so the end result is that the database contains Tom Jones not Joe Jones.

Tip: A block containing an EXCEPTION clause is significantly more expensive to enter and exit than a block without one. Therefore, don't use EXCEPTION without need.

Within an exception handler, the SQLSTATE variable contains the error code that corresponds to the exception that was raised (refer to Table A-1 for a list of possible error codes). The SQLERRM variable contains the error message associated with the exception. These variables are undefined outside exception handlers.

Example 37-1. Exceptions with UPDATE/INSERT

This example uses exception handling to perform either UPDATE or INSERT, as appropriate.

CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
    LOOP
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;

        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- do nothing
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');

Comments


March 8, 2007, 1:13 a.m.

One potential issue with the Insert/Update code above: a table lock will result in a never-ending loop. This code is cleaner:

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
DECLARE
i integer = 0;
BEGIN
LOOP
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;

BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing
END;

-- on repeated failure, abort and raise exception
IF i &gt; 10 THEN
Raise Exception 'Unable to perform insert / update on table db (%)', key;
END IF;
i = i + 1;
END LOOP;
END;
$$
LANGUAGE plpgsql;


Sept. 11, 2007, 3:15 p.m.

Robert Davidson <robdavid AT amazon.com> wrote in 8.1 doc
-------------
How to make RETURN NEXT was not clear to me until I fiddled around a bit. Here's hoping this is a timesaver for someone else:

CREATE TABLE test (textcol varchar(10), intcol int);
INSERT INTO test VALUES ('a', 1);
INSERT INTO test VALUES ('a', 2);
INSERT INTO test VALUES ('b', 5);
INSERT INTO test VALUES ('b', 6);

CREATE OR REPLACE FUNCTION ReturnNexting(pText Text) RETURNS SETOF test AS $$
DECLARE
rec RECORD;
BEGIN
FOR rec IN SELECT * FROM test WHERE textcol = pText LOOP
RETURN NEXT rec;
END LOOP;
RETURN;
END;
$$
LANGUAGE plpgsql;

SELECT * FROM ReturnNexting('a');
-----------------------------------
This amendment to get the most of return next.
if you have a row type like
CREATE mytype AS (name1 some_type,name2 some_type)
and you
RETURN NEXT mytype;
you can acces then like this
SELECT name1 AS some_alias_if_needed ,name2 FROM ReturnNexting('a');
--------------
Pg is great you discover nice features playing around trying to get things done


Dec. 13, 2007, 8:05 p.m.

I believe example 37-1. is flawed. The statement "This example uses exception handling to perform either UPDATE or INSERT, as appropriate." does not actually use exception handling to do anything. To wit:

EXCEPTION WHEN unique_violation THEN
-- do nothing
END;

This example is confusing and pointless. I read the code as an example of "Using FOUND to update of insert a record as appropriate."

The RETURN takes you out of the function either after a successful update, or after a successful insert. You could not have a duplicate insert because you would have already exited after the successful update.

The loop looks like it MIGHT cause an exception, except we exit before the END LOOP; is encountered the first time.

PROOF:
CREATE OR REPLACE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
RAISE INFO 'NOT DOING %','ANYTHING';
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;

The exception never raises, which means it plays no role in the process.

I believe the following code provides a better example:

CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
RAISE INFO 'UPDATING RECORD % WITH NEW DATA %',key, data;
UPDATE db SET b = data WHERE a = key;
RETURN;
END;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');


Jan. 15, 2008, 7:46 a.m.

In response to Matthew Peter's last comment.. He is incorrect, due to concurrency. The exception handling is necessary if there are multiple users accessing this database.

In the example, a row is first updated, and if that fails (row doesn't exist, it is inserted. Where the exception handling comes into play is if another user inserts the row between these two steps. It's possible your update can fail, and then your insert will also fail, because the row was inserted from another user.

The exception is caught, and the whole operation is tried again. This time, the update might succeed, since it was inserted. But if another user removed it, it would fail again. So you see, the point of the loop is to keep retrying this operation until the update or the delete succeeds.

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