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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>36.7.Control Structures</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="plpgsql-control-structures"></a>36.7.Control Structures</h2></div></div></div>
<p> Control structures are probably the most useful (and
important) part of <span class="application">PL/pgSQL</span>. With
<span class="application">PL/pgSQL</span>'s control structures,
you can manipulate <span class="productname">PostgreSQL</span> data in a very
flexible and powerful way.
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-statements-returning"></a>36.7.1.Returning From a Function</h3></div></div></div>
<p> There are two commands available that allow you to return data
from a function: <code class="command">RETURN</code> and <code class="command">RETURN
NEXT</code>.
</p>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724180"></a>36.7.1.1.<code class="command">RETURN</code></h4></div></div></div>
<pre class="synopsis">RETURN <em class="replaceable"><code>expression</code></em>;</pre>
<p> <code class="command">RETURN</code> with an expression terminates the
function and returns the value of
<em class="replaceable"><code>expression</code></em> to the caller. This form
is to be used for <span class="application">PL/pgSQL</span> functions that do
not return a set.
</p>
<p> 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 <em class="replaceable"><code>expression</code></em>.
</p>
<p> If you declared the function with output parameters, write just
<code class="command">RETURN</code> with no expression. The current values
of the output parameter variables will be returned.
</p>
<p> If you declared the function to return <code class="type">void</code>, a
<code class="command">RETURN</code> statement can be used to exit the function
early; but do not write an expression following
<code class="command">RETURN</code>.
</p>
<p> 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 <code class="command">RETURN</code> statement, a run-time
error will occur. This restriction does not apply to functions
with output parameters and functions returning <code class="type">void</code>,
however. In those cases a <code class="command">RETURN</code> statement is
automatically executed if the top-level block finishes.
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724292"></a>36.7.1.2.<code class="command">RETURN NEXT</code></h4></div></div></div>
<pre class="synopsis">RETURN NEXT <em class="replaceable"><code>expression</code></em>;</pre>
<p> When a <span class="application">PL/pgSQL</span> function is declared to return
<code class="literal">SETOF <em class="replaceable"><code>sometype</code></em></code>, the procedure
to follow is slightly different. In that case, the individual
items to return are specified in <code class="command">RETURN NEXT</code>
commands, and then a final <code class="command">RETURN</code> command
with no argument is used to indicate that the function has
finished executing. <code class="command">RETURN NEXT</code> can be used
with both scalar and composite data types; with a composite result
type, an entire “<span class="quote">table</span>” of results will be returned.
</p>
<p> <code class="command">RETURN NEXT</code> does not actually return from the
function [mdash ] it simply saves away the value of the expression.
Execution then continues with the next statement in
the <span class="application">PL/pgSQL</span> function. As successive
<code class="command">RETURN NEXT</code> commands are executed, the result
set is built up. A final <code class="command">RETURN</code>, which should
have no argument, causes control to exit the function (or you can
just let control reach the end of the function).
</p>
<p> If you declared the function with output parameters, write just
<code class="command">RETURN NEXT</code> 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
<code class="literal">SETOF record</code> when there are
multiple output parameters, or
<code class="literal">SETOF <em class="replaceable"><code>sometype</code></em></code> when there is
just one output parameter of type <em class="replaceable"><code>sometype</code></em>, in
order to create a set-returning function with output parameters.
</p>
<p> Functions that use <code class="command">RETURN NEXT</code> should be
called in the following fashion:
</p>
<pre class="programlisting">SELECT * FROM some_func();</pre>
<p>
That is, the function must be used as a table source in a
<code class="literal">FROM</code> clause.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p> The current implementation of <code class="command">RETURN NEXT</code>
for <span class="application">PL/pgSQL</span> stores the entire result set
before returning from the function, as discussed above. That
means that if a <span class="application">PL/pgSQL</span> 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 <span class="application">PL/pgSQL</span> 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
<a href="runtime-config-resource.html#guc-work-mem">work_mem</a>
configuration variable. Administrators who have sufficient
memory to store larger result sets in memory should consider
increasing this parameter.
</p>
</div>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-conditionals"></a>36.7.2.Conditionals</h3></div></div></div>
<p> <code class="literal">IF</code> statements let you execute commands based on
certain conditions. <span class="application">PL/pgSQL</span> has five forms of
<code class="literal">IF</code>:
</p>
<div class="itemizedlist"><ul type="disc">
<li><p><code class="literal">IF ... THEN</code></p></li>
<li><p><code class="literal">IF ... THEN ... ELSE</code></p></li>
<li><p><code class="literal">IF ... THEN ... ELSE IF</code></p></li>
<li><p><code class="literal">IF ... THEN ... ELSIF ... THEN ... ELSE</code></p></li>
<li><p><code class="literal">IF ... THEN ... ELSEIF ... THEN ... ELSE</code></p></li>
</ul></div>
<p>
</p>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724585"></a>36.7.2.1.<code class="literal">IF-THEN</code></h4></div></div></div>
<pre class="synopsis">IF <em class="replaceable"><code>boolean-expression</code></em> THEN
<em class="replaceable"><code>statements</code></em>
END IF;</pre>
<p> <code class="literal">IF-THEN</code> statements are the simplest form of
<code class="literal">IF</code>. The statements between
<code class="literal">THEN</code> and <code class="literal">END IF</code> will be
executed if the condition is true. Otherwise, they are
skipped.
</p>
<p> Example:
</p>
<pre class="programlisting">IF v_user_id <> 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724650"></a>36.7.2.2.<code class="literal">IF-THEN-ELSE</code></h4></div></div></div>
<pre class="synopsis">IF <em class="replaceable"><code>boolean-expression</code></em> THEN
<em class="replaceable"><code>statements</code></em>
ELSE
<em class="replaceable"><code>statements</code></em>
END IF;</pre>
<p> <code class="literal">IF-THEN-ELSE</code> statements add to
<code class="literal">IF-THEN</code> by letting you specify an
alternative set of statements that should be executed if the
condition evaluates to false.
</p>
<p> Examples:
</p>
<pre class="programlisting">IF parentid IS NULL OR parentid = ''
THEN
RETURN fullname;
ELSE
RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;</pre>
<p>
</p>
<pre class="programlisting">IF v_count > 0 THEN
INSERT INTO users_count (count) VALUES (v_count);
RETURN 't';
ELSE
RETURN 'f';
END IF;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724716"></a>36.7.2.3.<code class="literal">IF-THEN-ELSE IF</code></h4></div></div></div>
<p> <code class="literal">IF</code> statements can be nested, as in the
following example:
</p>
<pre class="programlisting">IF demo_row.sex = 'm' THEN
pretty_sex := 'man';
ELSE
IF demo_row.sex = 'f' THEN
pretty_sex := 'woman';
END IF;
END IF;</pre>
<p>
</p>
<p> When you use this form, you are actually nesting an
<code class="literal">IF</code> statement inside the
<code class="literal">ELSE</code> part of an outer <code class="literal">IF</code>
statement. Thus you need one <code class="literal">END IF</code>
statement for each nested <code class="literal">IF</code> and one for the parent
<code class="literal">IF-ELSE</code>. This is workable but grows
tedious when there are many alternatives to be checked.
Hence the next form.
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724789"></a>36.7.2.4.<code class="literal">IF-THEN-ELSIF-ELSE</code></h4></div></div></div>
<pre class="synopsis">IF <em class="replaceable"><code>boolean-expression</code></em> THEN
<em class="replaceable"><code>statements</code></em>
[<span class="optional"> ELSIF <em class="replaceable"><code>boolean-expression</code></em> THEN
<em class="replaceable"><code>statements</code></em>
[<span class="optional"> ELSIF <em class="replaceable"><code>boolean-expression</code></em> THEN
<em class="replaceable"><code>statements</code></em>
...</span>]</span>]
[<span class="optional"> ELSE
<em class="replaceable"><code>statements</code></em> </span>]
END IF;</pre>
<p> <code class="literal">IF-THEN-ELSIF-ELSE</code> provides a more convenient
method of checking many alternatives in one statement.
Formally it is equivalent to nested
<code class="literal">IF-THEN-ELSE-IF-THEN</code> commands, but only one
<code class="literal">END IF</code> is needed.
</p>
<p> Here is an example:
</p>
<pre class="programlisting">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;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724886"></a>36.7.2.5.<code class="literal">IF-THEN-ELSEIF-ELSE</code></h4></div></div></div>
<p> <code class="literal">ELSEIF</code> is an alias for <code class="literal">ELSIF</code>.
</p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-control-structures-loops"></a>36.7.3.Simple Loops</h3></div></div></div>
<a name="id724919"></a><p> With the <code class="literal">LOOP</code>, <code class="literal">EXIT</code>,
<code class="literal">CONTINUE</code>, <code class="literal">WHILE</code>, and <code class="literal">FOR</code>
statements, you can arrange for your <span class="application">PL/pgSQL</span>
function to repeat a series of commands.
</p>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id724977"></a>36.7.3.1.<code class="literal">LOOP</code></h4></div></div></div>
<pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
LOOP
<em class="replaceable"><code>statements</code></em>
END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>];</pre>
<p> <code class="literal">LOOP</code> defines an unconditional loop that is repeated
indefinitely until terminated by an <code class="literal">EXIT</code> or
<code class="command">RETURN</code> statement. The optional
<em class="replaceable"><code>label</code></em> can be used by <code class="literal">EXIT</code>
and <code class="literal">CONTINUE</code> statements in nested loops to
specify which loop the statement should be applied to.
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id725051"></a>36.7.3.2.<code class="literal">EXIT</code></h4></div></div></div>
<a name="id725058"></a><pre class="synopsis">EXIT [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>] [<span class="optional"> WHEN <em class="replaceable"><code>expression</code></em> </span>];</pre>
<p> If no <em class="replaceable"><code>label</code></em> is given, the innermost
loop is terminated and the statement following <code class="literal">END
LOOP</code> is executed next. If <em class="replaceable"><code>label</code></em>
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 <code class="literal">END</code>.
</p>
<p> If <code class="literal">WHEN</code> is specified, the loop exit occurs only if
<em class="replaceable"><code>expression</code></em> is true. Otherwise, control passes
to the statement after <code class="literal">EXIT</code>.
</p>
<p> <code class="literal">EXIT</code> can be used with all types of loops; it is
not limited to use with unconditional loops. When used with a
<code class="literal">BEGIN</code> block, <code class="literal">EXIT</code> passes
control to the next statement after the end of the block.
</p>
<p> Examples:
</p>
<pre class="programlisting">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;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id725181"></a>36.7.3.3.<code class="literal">CONTINUE</code></h4></div></div></div>
<a name="id725189"></a><pre class="synopsis">CONTINUE [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>] [<span class="optional"> WHEN <em class="replaceable"><code>expression</code></em> </span>];</pre>
<p> If no <em class="replaceable"><code>label</code></em> 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 <em class="replaceable"><code>label</code></em> is present, it
specifies the label of the loop whose execution will be
continued.
</p>
<p> If <code class="literal">WHEN</code> is specified, the next iteration of the
loop is begun only if <em class="replaceable"><code>expression</code></em> is
true. Otherwise, control passes to the statement after
<code class="literal">CONTINUE</code>.
</p>
<p> <code class="literal">CONTINUE</code> can be used with all types of loops; it
is not limited to use with unconditional loops.
</p>
<p> Examples:
</p>
<pre class="programlisting">LOOP
-- some computations
EXIT WHEN count > 100;
CONTINUE WHEN count < 50;
-- some computations for count IN [50 .. 100]
END LOOP;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id725283"></a>36.7.3.4.<code class="literal">WHILE</code></h4></div></div></div>
<a name="id725291"></a><pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
WHILE <em class="replaceable"><code>expression</code></em> LOOP
<em class="replaceable"><code>statements</code></em>
END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>];</pre>
<p> The <code class="literal">WHILE</code> 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.
</p>
<p> For example:
</p>
<pre class="programlisting">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;</pre>
<p>
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id725359"></a>36.7.3.5.<code class="literal">FOR</code> (integer variant)</h4></div></div></div>
<pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
FOR <em class="replaceable"><code>name</code></em> IN [<span class="optional"> REVERSE </span>] <em class="replaceable"><code>expression</code></em> .. <em class="replaceable"><code>expression</code></em> LOOP
<em class="replaceable"><code>statements</code></em>
END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>];</pre>
<p> This form of <code class="literal">FOR</code> creates a loop that iterates over a range of integer
values. The variable
<em class="replaceable"><code>name</code></em> is automatically defined as type
<code class="type">integer</code> and exists only inside the loop. The two expressions giving
the lower and upper bound of the range are evaluated once when entering
the loop. The iteration step is normally 1, but is -1 when <code class="literal">REVERSE</code> is
specified.
</p>
<p> Some examples of integer <code class="literal">FOR</code> loops:
</p>
<pre class="programlisting">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;</pre>
<p>
</p>
<p> If the lower bound is greater than the upper bound (or less than,
in the <code class="literal">REVERSE</code> case), the loop body is not
executed at all. No error is raised.
</p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-records-iterating"></a>36.7.4.Looping Through Query Results</h3></div></div></div>
<p> Using a different type of <code class="literal">FOR</code> loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
</p>
<pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
FOR <em class="replaceable"><code>record_or_row</code></em> IN <em class="replaceable"><code>query</code></em> LOOP
<em class="replaceable"><code>statements</code></em>
END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>];</pre>
<p>
The record or row variable is successively assigned each row
resulting from the <em class="replaceable"><code>query</code></em> (which must be a
<code class="command">SELECT</code> command) and the loop body is executed for each
row. Here is an example:
</p>
<pre class="programlisting">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;</pre>
<p>
If the loop is terminated by an <code class="literal">EXIT</code> statement, the last
assigned row value is still accessible after the loop.
</p>
<p> The <code class="literal">FOR-IN-EXECUTE</code> statement is another way to iterate over
rows:
</p>
<pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
FOR <em class="replaceable"><code>record_or_row</code></em> IN EXECUTE <em class="replaceable"><code>text_expression</code></em> LOOP
<em class="replaceable"><code>statements</code></em>
END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>];</pre>
<p>
This is like the previous form, except that the source
<code class="command">SELECT</code> statement is specified as a string
expression, which is evaluated and replanned on each entry to
the <code class="literal">FOR</code> 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 <code class="command">EXECUTE</code> statement.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p> The <span class="application">PL/pgSQL</span> parser presently distinguishes the
two kinds of <code class="literal">FOR</code> loops (integer or query result) by checking
whether <code class="literal">..</code> appears outside any parentheses between
<code class="literal">IN</code> and <code class="literal">LOOP</code>. If <code class="literal">..</code> is not seen then
the loop is presumed to be a loop over rows. Mistyping the <code class="literal">..</code>
is thus likely to lead to a complaint along the lines of
“<span class="quote">loop variable of loop over rows must be a record or row variable</span>”,
rather than the simple syntax error one might expect to get.
</p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-error-trapping"></a>36.7.5.Trapping Errors</h3></div></div></div>
<a name="id725701"></a><p> By default, any error occurring in a <span class="application">PL/pgSQL</span>
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 <code class="command">BEGIN</code> block with an
<code class="literal">EXCEPTION</code> clause. The syntax is an extension of the
normal syntax for a <code class="command">BEGIN</code> block:
</p>
<pre class="synopsis">[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>]
[<span class="optional"> DECLARE
<em class="replaceable"><code>declarations</code></em> </span>]
BEGIN
<em class="replaceable"><code>statements</code></em>
EXCEPTION
WHEN <em class="replaceable"><code>condition</code></em> [<span class="optional"> OR <em class="replaceable"><code>condition</code></em> ... </span>] THEN
<em class="replaceable"><code>handler_statements</code></em>
[<span class="optional"> WHEN <em class="replaceable"><code>condition</code></em> [<span class="optional"> OR <em class="replaceable"><code>condition</code></em> ... </span>] THEN
<em class="replaceable"><code>handler_statements</code></em>
... </span>]
END;</pre>
<p>
</p>
<p> If no error occurs, this form of block simply executes all the
<em class="replaceable"><code>statements</code></em>, and then control passes
to the next statement after <code class="literal">END</code>. But if an error
occurs within the <em class="replaceable"><code>statements</code></em>, further
processing of the <em class="replaceable"><code>statements</code></em> is
abandoned, and control passes to the <code class="literal">EXCEPTION</code> list.
The list is searched for the first <em class="replaceable"><code>condition</code></em>
matching the error that occurred. If a match is found, the
corresponding <em class="replaceable"><code>handler_statements</code></em> are
executed, and then control passes to the next statement after
<code class="literal">END</code>. If no match is found, the error propagates out
as though the <code class="literal">EXCEPTION</code> clause were not there at all:
the error can be caught by an enclosing block with
<code class="literal">EXCEPTION</code>, or if there is none it aborts processing
of the function.
</p>
<p> The <em class="replaceable"><code>condition</code></em> names can be any of
those shown in <a href="errcodes-appendix.html" title="AppendixA.PostgreSQL Error Codes">AppendixA, <i><span class="productname">PostgreSQL</span> Error Codes</i></a>. A category
name matches any error within its category. The special
condition name <code class="literal">OTHERS</code> matches every error type except
<code class="literal">QUERY_CANCELED</code>. (It is possible, but often unwise,
to trap <code class="literal">QUERY_CANCELED</code> by name.) Condition names are
not case-sensitive.
</p>
<p> If a new error occurs within the selected
<em class="replaceable"><code>handler_statements</code></em>, it cannot be caught
by this <code class="literal">EXCEPTION</code> clause, but is propagated out.
A surrounding <code class="literal">EXCEPTION</code> clause could catch it.
</p>
<p> When an error is caught by an <code class="literal">EXCEPTION</code> clause,
the local variables of the <span class="application">PL/pgSQL</span> 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:
</p>
<pre class="programlisting"> 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;</pre>
<p>
When control reaches the assignment to <code class="literal">y</code>, it will
fail with a <code class="literal">division_by_zero</code> error. This will be caught by
the <code class="literal">EXCEPTION</code> clause. The value returned in the
<code class="command">RETURN</code> statement will be the incremented value of
<code class="literal">x</code>, but the effects of the <code class="command">UPDATE</code> command will
have been rolled back. The <code class="command">INSERT</code> command preceding the
block is not rolled back, however, so the end result is that the database
contains <code class="literal">Tom Jones</code> not <code class="literal">Joe Jones</code>.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p> A block containing an <code class="literal">EXCEPTION</code> clause is significantly
more expensive to enter and exit than a block without one. Therefore,
don't use <code class="literal">EXCEPTION</code> without need.
</p>
</div>
<p> Within an exception handler, the <code class="varname">SQLSTATE</code>
variable contains the error code that corresponds to the
exception that was raised (refer to <a href="errcodes-appendix.html#errcodes-table" title="TableA.1.PostgreSQL Error Codes">TableA.1, “<span class="productname">PostgreSQL</span> Error Codes”</a> for a list of possible error
codes). The <code class="varname">SQLERRM</code> variable contains the
error message associated with the exception. These variables are
undefined outside exception handlers.
</p>
<div class="example">
<a name="plpgsql-upsert-example"></a><p class="title"><b>Example36.1.Exceptions with <code class="command">UPDATE</code>/<code class="command">INSERT</code></b></p>
<div class="example-contents">
<p>
This example uses exception handling to perform either
<code class="command">UPDATE</code> or <code class="command">INSERT</code>, as appropriate.
</p>
<pre class="programlisting">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');</pre>
<p>
</p>
</div>
</div>
<br class="example-break">
</div>
</div></body>
</html>
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