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postgresql9.4-docs-9.4.15-1.mga6.noarch.rpm

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>Chapter 40. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Procedural Language</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES"
>40.6. Control Structures</A
></H1
><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"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-STATEMENTS-RETURNING"
>40.6.1. Returning From a Function</A
></H2
><P
>     There are two commands available that allow you to return data
     from a function: <TT
CLASS="COMMAND"
>RETURN</TT
> and <TT
CLASS="COMMAND"
>RETURN
     NEXT</TT
>.
    </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN60898"
>40.6.1.1. <TT
CLASS="COMMAND"
>RETURN</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>RETURN <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>;</PRE
><P
>      <TT
CLASS="COMMAND"
>RETURN</TT
> with an expression terminates the
      function and returns the value of
      <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> to the caller.  This form
      is used for <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> functions that do
      not return a set.
     </P
><P
>      In a function that returns a scalar type, the expression's result will
      automatically be cast into the function's return type as described for
      assignments.  But to return a composite (row) value, you must write an
      expression delivering exactly the requested column set.  This may
      require use of explicit casting.
     </P
><P
>      If you declared the function with output parameters, write just
      <TT
CLASS="COMMAND"
>RETURN</TT
> with no expression.  The current values
      of the output parameter variables will be returned.
     </P
><P
>      If you declared the function to return <TT
CLASS="TYPE"
>void</TT
>, a
      <TT
CLASS="COMMAND"
>RETURN</TT
> statement can be used to exit the function
      early; but do not write an expression following
      <TT
CLASS="COMMAND"
>RETURN</TT
>.
     </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 <TT
CLASS="COMMAND"
>RETURN</TT
> statement, a run-time
      error will occur.  This restriction does not apply to functions
      with output parameters and functions returning <TT
CLASS="TYPE"
>void</TT
>,
      however.  In those cases a <TT
CLASS="COMMAND"
>RETURN</TT
> statement is
      automatically executed if the top-level block finishes.
     </P
><P
>      Some examples:

</P><PRE
CLASS="PROGRAMLISTING"
>-- functions returning a scalar type
RETURN 1 + 2;
RETURN scalar_var;

-- functions returning a composite type
RETURN composite_type_var;
RETURN (1, 2, 'three'::text);  -- must cast columns to correct types</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN60920"
>40.6.1.2. <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> and <TT
CLASS="COMMAND"
>RETURN QUERY</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>RETURN NEXT <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>;
RETURN QUERY <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
>;
RETURN QUERY EXECUTE <TT
CLASS="REPLACEABLE"
><I
>command-string</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> USING <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>] </SPAN
>];</PRE
><P
>      When a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function is declared to return
      <TT
CLASS="LITERAL"
>SETOF <TT
CLASS="REPLACEABLE"
><I
>sometype</I
></TT
></TT
>, the procedure
      to follow is slightly different.  In that case, the individual
      items to return are specified by a sequence of <TT
CLASS="COMMAND"
>RETURN
      NEXT</TT
> or <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> commands, and
      then a final <TT
CLASS="COMMAND"
>RETURN</TT
> command with no argument
      is used to indicate that the function has finished executing.
      <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> 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.
      <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> appends the results of executing
      a query to the function's result set. <TT
CLASS="COMMAND"
>RETURN
      NEXT</TT
> and <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> can be freely
      intermixed in a single set-returning function, in which case
      their results will be concatenated.
     </P
><P
>      <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> and <TT
CLASS="COMMAND"
>RETURN
      QUERY</TT
> do not actually return from the function &mdash;
      they simply append zero or more rows to the function's result
      set.  Execution then continues with the next statement in the
      <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function.  As successive
      <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> or <TT
CLASS="COMMAND"
>RETURN
      QUERY</TT
> commands are executed, the result set is built
      up.  A final <TT
CLASS="COMMAND"
>RETURN</TT
>, 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
>      <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> has a variant
      <TT
CLASS="COMMAND"
>RETURN QUERY EXECUTE</TT
>, which specifies the
      query to be executed dynamically.  Parameter expressions can
      be inserted into the computed query string via <TT
CLASS="LITERAL"
>USING</TT
>,
      in just the same way as in the <TT
CLASS="COMMAND"
>EXECUTE</TT
> command.
     </P
><P
>      If you declared the function with output parameters, write just
      <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> 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
      <TT
CLASS="LITERAL"
>SETOF record</TT
> when there are multiple output
      parameters, or <TT
CLASS="LITERAL"
>SETOF <TT
CLASS="REPLACEABLE"
><I
>sometype</I
></TT
></TT
>
      when there is just one output parameter of type
      <TT
CLASS="REPLACEABLE"
><I
>sometype</I
></TT
>, in order to create a set-returning
      function with output parameters.
     </P
><P
>      Here is an example of a function using <TT
CLASS="COMMAND"
>RETURN
      NEXT</TT
>:

</P><PRE
CLASS="PROGRAMLISTING"
>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 get_all_foo() RETURNS SETOF foo AS
$BODY$
DECLARE
    r foo%rowtype;
BEGIN
    FOR r IN
        SELECT * FROM foo WHERE fooid &gt; 0
    LOOP
        -- can do some processing here
        RETURN NEXT r; -- return current row of SELECT
    END LOOP;
    RETURN;
END
$BODY$
LANGUAGE plpgsql;

SELECT * FROM get_all_foo();</PRE
><P>
     </P
><P
>      Here is an example of a function using <TT
CLASS="COMMAND"
>RETURN
      QUERY</TT
>:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION get_available_flightid(date) RETURNS SETOF integer AS
$BODY$
BEGIN
    RETURN QUERY SELECT flightid
                   FROM flight
                  WHERE flightdate &#62;= $1
                    AND flightdate &#60; ($1 + 1);

    -- Since execution is not finished, we can check whether rows were returned
    -- and raise exception if not.
    IF NOT FOUND THEN
        RAISE EXCEPTION 'No flight at %.', $1;
    END IF;

    RETURN;
 END
$BODY$
LANGUAGE plpgsql;

-- Returns available flights or raises exception if there are no
-- available flights.
SELECT * FROM get_available_flightid(CURRENT_DATE);</PRE
><P>
     </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>       The current implementation of <TT
CLASS="COMMAND"
>RETURN NEXT</TT
>
       and <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> 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 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 <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> 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
       <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
></BLOCKQUOTE
></DIV
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CONDITIONALS"
>40.6.2. Conditionals</A
></H2
><P
>     <TT
CLASS="COMMAND"
>IF</TT
> and <TT
CLASS="COMMAND"
>CASE</TT
> statements let you execute
     alternative commands based on certain conditions.
     <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> has three forms of <TT
CLASS="COMMAND"
>IF</TT
>:
    <P
></P
></P><UL
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... END IF</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSE ... END IF</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF</TT
></P
></LI
></UL
><P>

    and two forms of <TT
CLASS="COMMAND"
>CASE</TT
>:
    <P
></P
></P><UL
><LI
><P
><TT
CLASS="LITERAL"
>CASE ... WHEN ... THEN ... ELSE ... END CASE</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>CASE WHEN ... THEN ... ELSE ... END CASE</TT
></P
></LI
></UL
><P>
    </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61005"
>40.6.2.1. <TT
CLASS="LITERAL"
>IF-THEN</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END IF;</PRE
><P
>        <TT
CLASS="LITERAL"
>IF-THEN</TT
> statements are the simplest form of
        <TT
CLASS="LITERAL"
>IF</TT
>. The statements between
        <TT
CLASS="LITERAL"
>THEN</TT
> and <TT
CLASS="LITERAL"
>END IF</TT
> will be
        executed if the condition is true. Otherwise, they are
        skipped.
       </P
><P
>        Example:
</P><PRE
CLASS="PROGRAMLISTING"
>IF v_user_id &lt;&gt; 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;</PRE
><P>
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61018"
>40.6.2.2. <TT
CLASS="LITERAL"
>IF-THEN-ELSE</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
ELSE
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END IF;</PRE
><P
>        <TT
CLASS="LITERAL"
>IF-THEN-ELSE</TT
> statements add to
        <TT
CLASS="LITERAL"
>IF-THEN</TT
> 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.)
       </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 &gt; 0 THEN
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61031"
>40.6.2.3. <TT
CLASS="LITERAL"
>IF-THEN-ELSIF</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
> ELSIF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
> ELSIF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
    ...</SPAN
>]</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
> ELSE
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> </SPAN
>]
END IF;</PRE
><P
>        Sometimes there are more than just two alternatives.
        <TT
CLASS="LITERAL"
>IF-THEN-ELSIF</TT
> provides a convenient
        method of checking several alternatives in turn.
        The <TT
CLASS="LITERAL"
>IF</TT
> 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 <TT
CLASS="LITERAL"
>END IF</TT
>.
        (Any subsequent <TT
CLASS="LITERAL"
>IF</TT
> conditions are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
        tested.)  If none of the <TT
CLASS="LITERAL"
>IF</TT
> conditions is true,
        then the <TT
CLASS="LITERAL"
>ELSE</TT
> block (if any) is executed.
       </P
><P
>        Here is an example:

</P><PRE
CLASS="PROGRAMLISTING"
>IF number = 0 THEN
    result := 'zero';
ELSIF number &gt; 0 THEN
    result := 'positive';
ELSIF number &lt; 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;</PRE
><P>
       </P
><P
>        The key word <TT
CLASS="LITERAL"
>ELSIF</TT
> can also be spelled
        <TT
CLASS="LITERAL"
>ELSEIF</TT
>.
       </P
><P
>        An alternative way of accomplishing the same task is to nest
        <TT
CLASS="LITERAL"
>IF-THEN-ELSE</TT
> statements, 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
>        However, this method requires writing a matching <TT
CLASS="LITERAL"
>END IF</TT
>
        for each <TT
CLASS="LITERAL"
>IF</TT
>, so it is much more cumbersome than
        using <TT
CLASS="LITERAL"
>ELSIF</TT
> when there are many alternatives.
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61065"
>40.6.2.4. Simple <TT
CLASS="LITERAL"
>CASE</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>CASE <TT
CLASS="REPLACEABLE"
><I
>search-expression</I
></TT
>
    WHEN <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> ... </SPAN
>]</SPAN
>] THEN
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
  [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> ... </SPAN
>]</SPAN
>] THEN
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
    ... </SPAN
>]
  [<SPAN
CLASS="OPTIONAL"
> ELSE
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> </SPAN
>]
END CASE;</PRE
><P
>       The simple form of <TT
CLASS="COMMAND"
>CASE</TT
> provides conditional execution
       based on equality of operands.  The <TT
CLASS="REPLACEABLE"
><I
>search-expression</I
></TT
>
       is evaluated (once) and successively compared to each
       <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> in the <TT
CLASS="LITERAL"
>WHEN</TT
> clauses.
       If a match is found, then the corresponding
       <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> are executed, and then control
       passes to the next statement after <TT
CLASS="LITERAL"
>END CASE</TT
>.  (Subsequent
       <TT
CLASS="LITERAL"
>WHEN</TT
> expressions are not evaluated.)  If no match is
       found, the <TT
CLASS="LITERAL"
>ELSE</TT
> <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> are
       executed; but if <TT
CLASS="LITERAL"
>ELSE</TT
> is not present, then a
       <TT
CLASS="LITERAL"
>CASE_NOT_FOUND</TT
> exception is raised.
      </P
><P
>       Here is a simple example:

</P><PRE
CLASS="PROGRAMLISTING"
>CASE x
    WHEN 1, 2 THEN
        msg := 'one or two';
    ELSE
        msg := 'other value than one or two';
END CASE;</PRE
><P>
      </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61097"
>40.6.2.5. Searched <TT
CLASS="LITERAL"
>CASE</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>CASE
    WHEN <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
  [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
    ... </SPAN
>]
  [<SPAN
CLASS="OPTIONAL"
> ELSE
      <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> </SPAN
>]
END CASE;</PRE
><P
>       The searched form of <TT
CLASS="COMMAND"
>CASE</TT
> provides conditional execution
       based on truth of Boolean expressions.  Each <TT
CLASS="LITERAL"
>WHEN</TT
> clause's
       <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> is evaluated in turn,
       until one is found that yields <TT
CLASS="LITERAL"
>true</TT
>.  Then the
       corresponding <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> are executed, and
       then control passes to the next statement after <TT
CLASS="LITERAL"
>END CASE</TT
>.
       (Subsequent <TT
CLASS="LITERAL"
>WHEN</TT
> expressions are not evaluated.)
       If no true result is found, the <TT
CLASS="LITERAL"
>ELSE</TT
>
       <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> are executed;
       but if <TT
CLASS="LITERAL"
>ELSE</TT
> is not present, then a
       <TT
CLASS="LITERAL"
>CASE_NOT_FOUND</TT
> exception is raised.
      </P
><P
>       Here is an example:

</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
      </P
><P
>       This form of <TT
CLASS="COMMAND"
>CASE</TT
> is entirely equivalent to
       <TT
CLASS="LITERAL"
>IF-THEN-ELSIF</TT
>, except for the rule that reaching
       an omitted <TT
CLASS="LITERAL"
>ELSE</TT
> clause results in an error rather
       than doing nothing.
      </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES-LOOPS"
>40.6.3. Simple Loops</A
></H2
><P
>     With the <TT
CLASS="LITERAL"
>LOOP</TT
>, <TT
CLASS="LITERAL"
>EXIT</TT
>,
     <TT
CLASS="LITERAL"
>CONTINUE</TT
>, <TT
CLASS="LITERAL"
>WHILE</TT
>, <TT
CLASS="LITERAL"
>FOR</TT
>,
     and <TT
CLASS="LITERAL"
>FOREACH</TT
> statements, you can arrange for your
     <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function to repeat a series of commands.
    </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61139"
>40.6.3.1. <TT
CLASS="LITERAL"
>LOOP</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P
>      <TT
CLASS="LITERAL"
>LOOP</TT
> defines an unconditional loop that is repeated
      indefinitely until terminated by an <TT
CLASS="LITERAL"
>EXIT</TT
> or
      <TT
CLASS="COMMAND"
>RETURN</TT
> statement.  The optional
      <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> can be used by <TT
CLASS="LITERAL"
>EXIT</TT
>
      and <TT
CLASS="LITERAL"
>CONTINUE</TT
> statements within nested loops to
      specify which loop those statements refer to.
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61155"
>40.6.3.2. <TT
CLASS="LITERAL"
>EXIT</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>EXIT [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> </SPAN
>];</PRE
><P
>        If no <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is given, the innermost
        loop is terminated and the statement following <TT
CLASS="LITERAL"
>END
        LOOP</TT
> is executed next.  If <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>
        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 <TT
CLASS="LITERAL"
>END</TT
>.
       </P
><P
>        If <TT
CLASS="LITERAL"
>WHEN</TT
> is specified, the loop exit occurs only if
        <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> is true. Otherwise, control passes
        to the statement after <TT
CLASS="LITERAL"
>EXIT</TT
>.
       </P
><P
>        <TT
CLASS="LITERAL"
>EXIT</TT
> can be used with all types of loops; it is
        not limited to use with unconditional loops.
       </P
><P
>        When used with a
        <TT
CLASS="LITERAL"
>BEGIN</TT
> block, <TT
CLASS="LITERAL"
>EXIT</TT
> passes
        control to the next statement after the end of the block.
        Note that a label must be used for this purpose; an unlabeled
        <TT
CLASS="LITERAL"
>EXIT</TT
> is never considered to match a
        <TT
CLASS="LITERAL"
>BEGIN</TT
> block.  (This is a change from
        pre-8.4 releases of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, which
        would allow an unlabeled <TT
CLASS="LITERAL"
>EXIT</TT
> to match
        a <TT
CLASS="LITERAL"
>BEGIN</TT
> block.)
       </P
><P
>        Examples:
</P><PRE
CLASS="PROGRAMLISTING"
>LOOP
    -- some computations
    IF count &gt; 0 THEN
        EXIT;  -- exit loop
    END IF;
END LOOP;

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

&lt;&lt;ablock&gt;&gt;
BEGIN
    -- some computations
    IF stocks &gt; 100000 THEN
        EXIT ablock;  -- causes exit from the BEGIN block
    END IF;
    -- computations here will be skipped when stocks &gt; 100000
END;</PRE
><P>
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61187"
>40.6.3.3. <TT
CLASS="LITERAL"
>CONTINUE</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>CONTINUE [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> </SPAN
>];</PRE
><P
>        If no <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> 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 <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is present, it
        specifies the label of the loop whose execution will be
        continued.
       </P
><P
>        If <TT
CLASS="LITERAL"
>WHEN</TT
> is specified, the next iteration of the
        loop is begun only if <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> is
        true. Otherwise, control passes to the statement after
        <TT
CLASS="LITERAL"
>CONTINUE</TT
>.
       </P
><P
>        <TT
CLASS="LITERAL"
>CONTINUE</TT
> 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 &gt; 100;
    CONTINUE WHEN count &lt; 50;
    -- some computations for count IN [50 .. 100]
END LOOP;</PRE
><P>
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN61209"
>40.6.3.4. <TT
CLASS="LITERAL"
>WHILE</TT
></A
></H3
><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
WHILE <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P
>        The <TT
CLASS="LITERAL"
>WHILE</TT
> statement repeats a
        sequence of statements so long as the
        <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
>
        evaluates to true.  The expression is checked just before
        each entry to the loop body.
       </P
><P
>        For example:
</P><PRE
CLASS="PROGRAMLISTING"
>WHILE amount_owed &gt; 0 AND gift_certificate_balance &gt; 0 LOOP
    -- some computations here
END LOOP;

WHILE NOT done LOOP
    -- some computations here
END LOOP;</PRE
><P>
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="PLPGSQL-INTEGER-FOR"
>40.6.3.5. <TT
CLASS="LITERAL"
>FOR</TT
> (Integer Variant)</A
></H3
><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> IN [<SPAN
CLASS="OPTIONAL"
> REVERSE </SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> .. <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> BY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> </SPAN
>] LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P
>        This form of <TT
CLASS="LITERAL"
>FOR</TT
> creates a loop that iterates over a range
        of integer values. The variable
        <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> is automatically defined as type
        <TT
CLASS="TYPE"
>integer</TT
> 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 <TT
CLASS="LITERAL"
>BY</TT
> clause isn't specified the iteration
        step is 1, otherwise it's the value specified in the <TT
CLASS="LITERAL"
>BY</TT
>
        clause, which again is evaluated once on loop entry.
        If <TT
CLASS="LITERAL"
>REVERSE</TT
> is specified then the step value is
        subtracted, rather than added, after each iteration.
       </P
><P
>        Some examples of integer <TT
CLASS="LITERAL"
>FOR</TT
> loops:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
       </P
><P
>        If the lower bound is greater than the upper bound (or less than,
        in the <TT
CLASS="LITERAL"
>REVERSE</TT
> case), the loop body is not
        executed at all.  No error is raised.
       </P
><P
>        If a <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is attached to the
        <TT
CLASS="LITERAL"
>FOR</TT
> loop then the integer loop variable can be
        referenced with a qualified name, using that
        <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>.
       </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-RECORDS-ITERATING"
>40.6.4. Looping Through Query Results</A
></H2
><P
>     Using a different type of <TT
CLASS="LITERAL"
>FOR</TT
> 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"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> IN <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
> LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P>
     The <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> is a record variable, row variable,
     or comma-separated list of scalar variables.
     The <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> is successively assigned each row
     resulting from the <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
> 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
    RAISE NOTICE '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

        RAISE NOTICE 'Refreshing materialized view %s ...', 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;

    RAISE NOTICE 'Done refreshing materialized views.';
    RETURN 1;
END;
$$ LANGUAGE plpgsql;</PRE
><P>

     If the loop is terminated by an <TT
CLASS="LITERAL"
>EXIT</TT
> statement, the last
     assigned row value is still accessible after the loop.
    </P
><P
>     The <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
> used in this type of <TT
CLASS="LITERAL"
>FOR</TT
>
     statement can be any SQL command that returns rows to the caller:
     <TT
CLASS="COMMAND"
>SELECT</TT
> is the most common case,
     but you can also use <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
     <TT
CLASS="COMMAND"
>DELETE</TT
> with a <TT
CLASS="LITERAL"
>RETURNING</TT
> clause.  Some utility
     commands such as <TT
CLASS="COMMAND"
>EXPLAIN</TT
> will work too.
    </P
><P
>     <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> variables are substituted into the query text,
     and the query plan is cached for possible re-use, as discussed in
     detail in <A
HREF="plpgsql-implementation.html#PLPGSQL-VAR-SUBST"
>Section 40.10.1</A
> and
     <A
HREF="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING"
>Section 40.10.2</A
>.
    </P
><P
>     The <TT
CLASS="LITERAL"
>FOR-IN-EXECUTE</TT
> statement is another way to iterate over
     rows:
</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> IN EXECUTE <TT
CLASS="REPLACEABLE"
><I
>text_expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> USING <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>] </SPAN
>] LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P>
     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 <TT
CLASS="LITERAL"
>FOR</TT
> 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 <TT
CLASS="COMMAND"
>EXECUTE</TT
> statement.
     As with <TT
CLASS="COMMAND"
>EXECUTE</TT
>, parameter values can be inserted
     into the dynamic command via <TT
CLASS="LITERAL"
>USING</TT
>.
    </P
><P
>     Another way to specify the query whose results should be iterated
     through is to declare it as a cursor.  This is described in
     <A
HREF="plpgsql-cursors.html#PLPGSQL-CURSOR-FOR-LOOP"
>Section 40.7.4</A
>.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-FOREACH-ARRAY"
>40.6.5. Looping Through Arrays</A
></H2
><P
>     The <TT
CLASS="LITERAL"
>FOREACH</TT
> loop is much like a <TT
CLASS="LITERAL"
>FOR</TT
> loop,
     but instead of iterating through the rows returned by a SQL query,
     it iterates through the elements of an array value.
     (In general, <TT
CLASS="LITERAL"
>FOREACH</TT
> is meant for looping through
     components of a composite-valued expression; variants for looping
     through composites besides arrays may be added in future.)
     The <TT
CLASS="LITERAL"
>FOREACH</TT
> statement to loop over an array is:

</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
FOREACH <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> SLICE <TT
CLASS="REPLACEABLE"
><I
>number</I
></TT
> </SPAN
>] IN ARRAY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> LOOP
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P>
    </P
><P
>     Without <TT
CLASS="LITERAL"
>SLICE</TT
>, or if <TT
CLASS="LITERAL"
>SLICE 0</TT
> is specified,
     the loop iterates through individual elements of the array produced
     by evaluating the <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>.
     The <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> variable is assigned each
     element value in sequence, and the loop body is executed for each element.
     Here is an example of looping through the elements of an integer
     array:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION sum(int[]) RETURNS int8 AS $$
DECLARE
  s int8 := 0;
  x int;
BEGIN
  FOREACH x IN ARRAY $1
  LOOP
    s := s + x;
  END LOOP;
  RETURN s;
END;
$$ LANGUAGE plpgsql;</PRE
><P>

     The elements are visited in storage order, regardless of the number of
     array dimensions.  Although the <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> is
     usually just a single variable, it can be a list of variables when
     looping through an array of composite values (records).  In that case,
     for each array element, the variables are assigned from successive
     columns of the composite value.
    </P
><P
>     With a positive <TT
CLASS="LITERAL"
>SLICE</TT
> value, <TT
CLASS="LITERAL"
>FOREACH</TT
>
     iterates through slices of the array rather than single elements.
     The <TT
CLASS="LITERAL"
>SLICE</TT
> value must be an integer constant not larger
     than the number of dimensions of the array.  The
     <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> variable must be an array,
     and it receives successive slices of the array value, where each slice
     is of the number of dimensions specified by <TT
CLASS="LITERAL"
>SLICE</TT
>.
     Here is an example of iterating through one-dimensional slices:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION scan_rows(int[]) RETURNS void AS $$
DECLARE
  x int[];
BEGIN
  FOREACH x SLICE 1 IN ARRAY $1
  LOOP
    RAISE NOTICE 'row = %', x;
  END LOOP;
END;
$$ LANGUAGE plpgsql;

SELECT scan_rows(ARRAY[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]);

NOTICE:  row = {1,2,3}
NOTICE:  row = {4,5,6}
NOTICE:  row = {7,8,9}
NOTICE:  row = {10,11,12}</PRE
><P>
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-ERROR-TRAPPING"
>40.6.6. Trapping Errors</A
></H2
><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 <TT
CLASS="COMMAND"
>BEGIN</TT
> block with an
     <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause.  The syntax is an extension of the
     normal syntax for a <TT
CLASS="COMMAND"
>BEGIN</TT
> block:

</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
[<SPAN
CLASS="OPTIONAL"
> DECLARE
    <TT
CLASS="REPLACEABLE"
><I
>declarations</I
></TT
> </SPAN
>]
BEGIN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
EXCEPTION
    WHEN <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> OR <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> ... </SPAN
>] THEN
        <TT
CLASS="REPLACEABLE"
><I
>handler_statements</I
></TT
>
    [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> OR <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> ... </SPAN
>] THEN
          <TT
CLASS="REPLACEABLE"
><I
>handler_statements</I
></TT
>
      ... </SPAN
>]
END;</PRE
><P>
    </P
><P
>     If no error occurs, this form of block simply executes all the
     <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>, and then control passes
     to the next statement after <TT
CLASS="LITERAL"
>END</TT
>.  But if an error
     occurs within the <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>, further
     processing of the <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> is
     abandoned, and control passes to the <TT
CLASS="LITERAL"
>EXCEPTION</TT
> list.
     The list is searched for the first <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
>
     matching the error that occurred.  If a match is found, the
     corresponding <TT
CLASS="REPLACEABLE"
><I
>handler_statements</I
></TT
> are
     executed, and then control passes to the next statement after
     <TT
CLASS="LITERAL"
>END</TT
>.  If no match is found, the error propagates out
     as though the <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause were not there at all:
     the error can be caught by an enclosing block with
     <TT
CLASS="LITERAL"
>EXCEPTION</TT
>, or if there is none it aborts processing
     of the function.
    </P
><P
>     The <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> names can be any of
     those shown in <A
HREF="errcodes-appendix.html"
>Appendix A</A
>.  A category
     name matches any error within its category.  The special
     condition name <TT
CLASS="LITERAL"
>OTHERS</TT
> matches every error type except
     <TT
CLASS="LITERAL"
>QUERY_CANCELED</TT
>.  (It is possible, but often unwise,
     to trap <TT
CLASS="LITERAL"
>QUERY_CANCELED</TT
> by name.)  Condition names are
     not case-sensitive.  Also, an error condition can be specified
     by <TT
CLASS="LITERAL"
>SQLSTATE</TT
> code; for example these are equivalent:
</P><PRE
CLASS="PROGRAMLISTING"
>WHEN division_by_zero THEN ...
WHEN SQLSTATE '22012' THEN ...</PRE
><P>
    </P
><P
>     If a new error occurs within the selected
     <TT
CLASS="REPLACEABLE"
><I
>handler_statements</I
></TT
>, it cannot be caught
     by this <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause, but is propagated out.
     A surrounding <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause could catch it.
    </P
><P
>     When an error is caught by an <TT
CLASS="LITERAL"
>EXCEPTION</TT
> 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 <TT
CLASS="LITERAL"
>y</TT
>, it will
     fail with a <TT
CLASS="LITERAL"
>division_by_zero</TT
> error.  This will be caught by
     the <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause.  The value returned in the
     <TT
CLASS="COMMAND"
>RETURN</TT
> statement will be the incremented value of
     <TT
CLASS="LITERAL"
>x</TT
>, but the effects of the <TT
CLASS="COMMAND"
>UPDATE</TT
> command will
     have been rolled back.  The <TT
CLASS="COMMAND"
>INSERT</TT
> command preceding the
     block is not rolled back, however, so the end result is that the database
     contains <TT
CLASS="LITERAL"
>Tom Jones</TT
> not <TT
CLASS="LITERAL"
>Joe Jones</TT
>.
    </P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>      A block containing an <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause is significantly
      more expensive to enter and exit than a block without one.  Therefore,
      don't use <TT
CLASS="LITERAL"
>EXCEPTION</TT
> without need.
     </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="PLPGSQL-UPSERT-EXAMPLE"
></A
><P
><B
>Example 40-2. Exceptions with <TT
CLASS="COMMAND"
>UPDATE</TT
>/<TT
CLASS="COMMAND"
>INSERT</TT
></B
></P
><P
>&#13;    This example uses exception handling to perform either
    <TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>INSERT</TT
>, 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
$$
BEGIN
    LOOP
        -- first try to update the key
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;
        -- not there, so try to insert the key
        -- if someone else inserts the same key concurrently,
        -- we could get a unique-key failure
        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- Do nothing, and loop to try the UPDATE again.
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');</PRE
><P>

     This coding assumes the <TT
CLASS="LITERAL"
>unique_violation</TT
> error is caused by
     the <TT
CLASS="COMMAND"
>INSERT</TT
>, and not by, say, an <TT
CLASS="COMMAND"
>INSERT</TT
> in a
     trigger function on the table.  It might also misbehave if there is
     more than one unique index on the table, since it will retry the
     operation regardless of which index caused the error.
     More safety could be had by using the
     features discussed next to check that the trapped error was the one
     expected.
    </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="PLPGSQL-EXCEPTION-DIAGNOSTICS"
>40.6.6.1. Obtaining Information About an Error</A
></H3
><P
>     Exception handlers frequently need to identify the specific error that
     occurred.  There are two ways to get information about the current
     exception in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>: special variables and the
     <TT
CLASS="COMMAND"
>GET STACKED DIAGNOSTICS</TT
> command.
    </P
><P
>     Within an exception handler, the special variable
     <TT
CLASS="VARNAME"
>SQLSTATE</TT
> contains the error code that corresponds to
     the exception that was raised (refer to <A
HREF="errcodes-appendix.html#ERRCODES-TABLE"
>Table A-1</A
>
     for a list of possible error codes). The special variable
     <TT
CLASS="VARNAME"
>SQLERRM</TT
> contains the error message associated with the
     exception. These variables are undefined outside exception handlers.
    </P
><P
>     Within an exception handler, one may also retrieve
     information about the current exception by using the
     <TT
CLASS="COMMAND"
>GET STACKED DIAGNOSTICS</TT
> command, which has the form:

</P><PRE
CLASS="SYNOPSIS"
>GET STACKED DIAGNOSTICS <TT
CLASS="REPLACEABLE"
><I
>variable</I
></TT
> { = | := } <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> , ... </SPAN
>];</PRE
><P>

     Each <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
> is a key word identifying a status
     value to be assigned to the specified <TT
CLASS="REPLACEABLE"
><I
>variable</I
></TT
>
     (which should be of the right data type to receive it).  The currently
     available status items are shown
     in <A
HREF="plpgsql-control-structures.html#PLPGSQL-EXCEPTION-DIAGNOSTICS-VALUES"
>Table 40-2</A
>.
    </P
><DIV
CLASS="TABLE"
><A
NAME="PLPGSQL-EXCEPTION-DIAGNOSTICS-VALUES"
></A
><P
><B
>Table 40-2. Error Diagnostics Items</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
>RETURNED_SQLSTATE</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the SQLSTATE error code of the exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>COLUMN_NAME</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the name of the column related to exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>CONSTRAINT_NAME</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the name of the constraint related to exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>PG_DATATYPE_NAME</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the name of the data type related to exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>MESSAGE_TEXT</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the text of the exception's primary message</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>TABLE_NAME</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the name of the table related to exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>SCHEMA_NAME</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the name of the schema related to exception</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>PG_EXCEPTION_DETAIL</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the text of the exception's detail message, if any</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>PG_EXCEPTION_HINT</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>the text of the exception's hint message, if any</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>PG_EXCEPTION_CONTEXT</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>line(s) of text describing the call stack at the time of the
          exception (see <A
HREF="plpgsql-control-structures.html#PLPGSQL-CALL-STACK"
>Section 40.6.7</A
>)</TD
></TR
></TBODY
></TABLE
></DIV
><P
>     If the exception did not set a value for an item, an empty string
     will be returned.
    </P
><P
>     Here is an example:
</P><PRE
CLASS="PROGRAMLISTING"
>DECLARE
  text_var1 text;
  text_var2 text;
  text_var3 text;
BEGIN
  -- some processing which might cause an exception
  ...
EXCEPTION WHEN OTHERS THEN
  GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                          text_var2 = PG_EXCEPTION_DETAIL,
                          text_var3 = PG_EXCEPTION_HINT;
END;</PRE
><P>
    </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CALL-STACK"
>40.6.7. Obtaining Execution Location Information</A
></H2
><P
>    The <TT
CLASS="COMMAND"
>GET DIAGNOSTICS</TT
> command, previously described
    in <A
HREF="plpgsql-statements.html#PLPGSQL-STATEMENTS-DIAGNOSTICS"
>Section 40.5.5</A
>, retrieves information
    about current execution state (whereas the <TT
CLASS="COMMAND"
>GET STACKED
    DIAGNOSTICS</TT
> command discussed above reports information about
    the execution state as of a previous error).  Its <TT
CLASS="LITERAL"
>PG_CONTEXT</TT
>
    status item is useful for identifying the current execution
    location.  <TT
CLASS="LITERAL"
>PG_CONTEXT</TT
> returns a text string with line(s)
    of text describing the call stack.  The first line refers to the current
    function and currently executing <TT
CLASS="COMMAND"
>GET DIAGNOSTICS</TT
>
    command.  The second and any subsequent lines refer to calling functions
    further up the call stack.  For example:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION outer_func() RETURNS integer AS $$
BEGIN
  RETURN inner_func();
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION inner_func() RETURNS integer AS $$
DECLARE
  stack text;
BEGIN
  GET DIAGNOSTICS stack = PG_CONTEXT;
  RAISE NOTICE E'--- Call Stack ---\n%', stack;
  RETURN 1;
END;
$$ LANGUAGE plpgsql;

SELECT outer_func();

NOTICE:  --- Call Stack ---
PL/pgSQL function inner_func() line 5 at GET DIAGNOSTICS
PL/pgSQL function outer_func() line 3 at RETURN
CONTEXT:  PL/pgSQL function outer_func() line 3 at RETURN
 outer_func
 ------------
           1
(1 row)</PRE
><P>

   </P
><P
>    <TT
CLASS="LITERAL"
>GET STACKED DIAGNOSTICS ... PG_EXCEPTION_CONTEXT</TT
>
    returns the same sort of stack trace, but describing the location
    at which an error was detected, rather than the current location.
   </P
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