Control Structures

PostgreSQL 8.4.2 Documentation
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# 38.6. 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.

## 38.6.1. Returning From a Function

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

### 38.6.1.1. `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.

### 38.6.1.2. `RETURN NEXT` and `RETURN QUERY`

    RETURN NEXT expression;
    RETURN QUERY query;
    RETURN QUERY EXECUTE command-string [ USING 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 by a sequence of `RETURN NEXT` or `RETURN QUERY` 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 QUERY` appends the results of executing a query to the function's result set. `RETURN NEXT` and `RETURN QUERY` can be freely intermixed in a single set-returning function, in which case their results will be concatenated.

`RETURN NEXT` and `RETURN QUERY` do not actually return from the function — they simply append zero or more rows to the function's result set. Execution then continues with the next statement in the PL/pgSQL function. As successive `RETURN NEXT` or `RETURN QUERY` 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).

`RETURN QUERY` has a variant `RETURN QUERY EXECUTE`, which specifies the query to be executed dynamically. Parameter expressions can be inserted into the computed query string via `USING`, in just the same way as in the `EXECUTE` command.

If you declared the function with output parameters, write just `RETURN NEXT` with no expression. On each execution, the current values of the output parameter variable(s) will be saved for eventual return as a row of the result. 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.

Here is an example of a function using `RETURN NEXT`:

    CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
    INSERT INTO foo VALUES (1, 2, 'three');
    INSERT INTO foo VALUES (4, 5, 'six');

    CREATE OR REPLACE FUNCTION getAllFoo() RETURNS SETOF foo AS
    $BODY$
    DECLARE
        r foo%rowtype;
    BEGIN
        FOR r IN SELECT * FROM foo
        WHERE fooid > 0
        LOOP
            -- can do some processing here
            RETURN NEXT r; -- return current row of SELECT
        END LOOP;
        RETURN;
    END
    $BODY$
    LANGUAGE 'plpgsql' ;

    SELECT * FROM getallfoo();

> Note: The current implementation of `RETURN NEXT` and `RETURN QUERY` 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 might 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 might 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.

## 38.6.2. Conditionals

`IF` and `CASE` statements let you execute alternative commands based on certain conditions. PL/pgSQL has three forms of `IF`:

- `IF ... THEN`

- `IF ... THEN ... ELSE`

- `IF ... THEN ... ELSIF ... THEN ... ELSE`

and two forms of `CASE`:

- `CASE ... WHEN ... THEN ... ELSE ... END CASE`

- `CASE WHEN ... THEN ... ELSE ... END CASE`

### 38.6.2.1. `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;

### 38.6.2.2. `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 is not true. (Note this includes the case where the condition evaluates to NULL.)

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;

### 38.6.2.3. `IF-THEN-ELSIF`

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

Sometimes there are more than just two alternatives. `IF-THEN-ELSIF` provides a convenient method of checking several alternatives in turn. The `IF` conditions are tested successively until the first one that is true is found. Then the associated statement(s) are executed, after which control passes to the next statement after `END IF`. (Any subsequent `IF` conditions are not tested.) If none of the `IF` conditions is true, then the `ELSE` block (if any) is executed.

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;

The key word `ELSIF` can also be spelled `ELSEIF`.

An alternative way of accomplishing the same task is to nest `IF-THEN-ELSE` statements, 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;

However, this method requires writing a matching `END IF` for each `IF`, so it is much more cumbersome than using `ELSIF` when there are many alternatives.

### 38.6.2.4. Simple `CASE`

    CASE search-expression
        WHEN expression [, expression [ ... ]] THEN
          statements
      [ WHEN expression [, expression [ ... ]] THEN
          statements
        ... ]
      [ ELSE
          statements ]
    END CASE;

The simple form of `CASE` provides conditional execution based on equality of operands. The `search-expression` is evaluated (once) and successively compared to each `expression` in the `WHEN` clauses. If a match is found, then the corresponding `statements` are executed, and then control passes to the next statement after `END CASE`. (Subsequent `WHEN` expressions are not evaluated.) If no match is found, the `ELSE` `statements` are executed; but if `ELSE` is not present, then a `CASE_NOT_FOUND` exception is raised.

Here is a simple example:

    CASE x
        WHEN 1, 2 THEN
            msg := 'one or two';
        ELSE
            msg := 'other value than one or two';
    END CASE;

### 38.6.2.5. Searched `CASE`

    CASE
        WHEN boolean-expression THEN
          statements
      [ WHEN boolean-expression THEN
          statements
        ... ]
      [ ELSE
          statements ]
    END CASE;

The searched form of `CASE` provides conditional execution based on truth of boolean expressions. Each `WHEN` clause's `boolean-expression` is evaluated in turn, until one is found that yields `true`. Then the corresponding `statements` are executed, and then control passes to the next statement after `END CASE`. (Subsequent `WHEN` expressions are not evaluated.) If no true result is found, the `ELSE` `statements` are executed; but if `ELSE` is not present, then a `CASE_NOT_FOUND` exception is raised.

Here is an example:

    CASE
        WHEN x BETWEEN 0 AND 10 THEN
            msg := 'value is between zero and ten';
        WHEN x BETWEEN 11 AND 20 THEN
            msg := 'value is between eleven and twenty';
    END CASE;

This form of `CASE` is entirely equivalent to `IF-THEN-ELSIF`, except for the rule that reaching an omitted `ELSE` clause results in an error rather than doing nothing.

## 38.6.3. 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.

### 38.6.3.1. `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 within nested loops to specify which loop those statements refer to.

### 38.6.3.2. `EXIT`

    EXIT [ label ] [ WHEN boolean-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 `boolean-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. Note that a label must be used for this purpose; an unlabelled `EXIT` is never considered to match a `BEGIN` block. (This is a change from pre-8.4 releases of PostgreSQL, which would allow an unlabelled `EXIT` to match a `BEGIN` 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;

    <<ablock>>
    BEGIN
        -- some computations
        IF stocks > 100000 THEN
            EXIT ablock;  -- causes exit from the BEGIN block
        END IF;
        -- computations here will be skipped when stocks > 100000
    END;

### 38.6.3.3. `CONTINUE`

    CONTINUE [ label ] [ WHEN boolean-expression ];

If no `label` is given, the next iteration of the innermost loop is begun. That is, all statements remaining in the loop body are skipped, and control returns to the loop control expression (if any) to determine whether another loop iteration is needed. 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 `boolean-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;

### 38.6.3.4. `WHILE`

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

The `WHILE` statement repeats a sequence of statements so long as the `boolean-expression` evaluates to true. The expression 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 done LOOP
        -- some computations here
    END LOOP;

### 38.6.3.5. `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, which again is evaluated once on loop entry. If `REVERSE` is specified then the step value is subtracted, rather than added, after each iteration.

Some examples of integer `FOR` loops:

    FOR i IN 1..10 LOOP
        -- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 LOOP
        -- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 BY 2 LOOP
        -- i will take on the values 10,8,6,4,2 within the loop
    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.

If a `label` is attached to the `FOR` loop then the integer loop variable can be referenced with a qualified name, using that `label`.

## 38.6.4. 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.

PL/pgSQL variables are substituted into the query text, and the query plan is cached for possible re-use, as discussed in detail in Section 38.10.1 and Section 38.10.2.

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

    [ <<label>> ]
    FOR target IN EXECUTE text_expression [ USING 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. As with `EXECUTE`, parameter values can be inserted into the dynamic command via `USING`.

Another way to specify the query whose results should be iterated through is to declare it as a cursor. This is described in Section 38.7.4.

## 38.6.5. 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. Also, an error condition can be specified by `SQLSTATE` code; for example these are equivalent:

            WHEN division_by_zero THEN ...
            WHEN SQLSTATE '22012' THEN ...

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 38-2. 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
    $$
    BEGIN
        LOOP
            -- first try to update the key
            UPDATE db SET b = data WHERE a = key;
            IF found THEN
                RETURN;
            END IF;
            -- not there, so try to insert the key
            -- if someone else inserts the same key concurrently,
            -- we could get a unique-key failure
            BEGIN
                INSERT INTO db(a,b) VALUES (key, data);
                RETURN;
            EXCEPTION WHEN unique_violation THEN
                -- do nothing, and loop to try the UPDATE again
            END;
        END LOOP;
    END;
    $$
    LANGUAGE plpgsql;

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

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