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postgresql8.3-docs-8.3.6-2mdv2008.1.x86_64.rpm

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>Chapter 38. <SPAN
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> - <ACRONYM
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> Procedural Language</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES"
>38.6. 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"
>38.6.1. 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="AEN43815"
>38.6.1.1. <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 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 <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>.
     </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
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN43836"
>38.6.1.2. <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> and <TT
CLASS="COMMAND"
>RETURN QUERY</TT
></A
></H3
><A
NAME="AEN43840"
></A
><A
NAME="AEN43843"
></A
><PRE
CLASS="SYNOPSIS"
>RETURN NEXT <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>;
RETURN QUERY <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
>;</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
>      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 getAllFoo() 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 getallfoo();</PRE
><P>

      Note that functions using <TT
CLASS="COMMAND"
>RETURN NEXT</TT
> or
      <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> must be called as a table source in
      a <TT
CLASS="LITERAL"
>FROM</TT
> clause.

     </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"
>38.6.2. Conditionals</A
></H2
><P
>     <TT
CLASS="LITERAL"
>IF</TT
> statements let you execute commands based on
     certain conditions.  <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> has five forms of
     <TT
CLASS="LITERAL"
>IF</TT
>:
    <P
></P
></P><UL
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSE</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSE IF</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSIF ... THEN ... ELSE</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSEIF ... THEN ... ELSE</TT
></P
></LI
></UL
><P>
    </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN43909"
>38.6.2.1. <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="AEN43922"
>38.6.2.2. <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 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 &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="AEN43935"
>38.6.2.3. <TT
CLASS="LITERAL"
>IF-THEN-ELSE IF</TT
></A
></H3
><P
>        <TT
CLASS="LITERAL"
>IF</TT
> 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
        <TT
CLASS="LITERAL"
>IF</TT
> statement inside the
        <TT
CLASS="LITERAL"
>ELSE</TT
> part of an outer <TT
CLASS="LITERAL"
>IF</TT
>
        statement. Thus you need one <TT
CLASS="LITERAL"
>END IF</TT
>
        statement for each nested <TT
CLASS="LITERAL"
>IF</TT
> and one for the parent
        <TT
CLASS="LITERAL"
>IF-ELSE</TT
>.  This is workable but grows
        tedious when there are many alternatives to be checked.
        Hence the next form.
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN43948"
>38.6.2.4. <TT
CLASS="LITERAL"
>IF-THEN-ELSIF-ELSE</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
>        <TT
CLASS="LITERAL"
>IF-THEN-ELSIF-ELSE</TT
> provides a more convenient
        method of checking many alternatives in one statement.
        Functionally it is equivalent to nested
        <TT
CLASS="LITERAL"
>IF-THEN-ELSE-IF-THEN</TT
> commands, but only one
        <TT
CLASS="LITERAL"
>END IF</TT
> is needed.
       </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
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN43968"
>38.6.2.5. <TT
CLASS="LITERAL"
>IF-THEN-ELSEIF-ELSE</TT
></A
></H3
><P
>       <TT
CLASS="LITERAL"
>ELSEIF</TT
> is an alias for <TT
CLASS="LITERAL"
>ELSIF</TT
>.
      </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES-LOOPS"
>38.6.3. Simple Loops</A
></H2
><A
NAME="AEN43976"
></A
><P
>     With the <TT
CLASS="LITERAL"
>LOOP</TT
>, <TT
CLASS="LITERAL"
>EXIT</TT
>,
     <TT
CLASS="LITERAL"
>CONTINUE</TT
>, <TT
CLASS="LITERAL"
>WHILE</TT
>, and <TT
CLASS="LITERAL"
>FOR</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="AEN43986"
>38.6.3.1. <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 in nested loops to
      specify which loop the statement should be applied to.
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN44002"
>38.6.3.2. <TT
CLASS="LITERAL"
>EXIT</TT
></A
></H3
><A
NAME="AEN44005"
></A
><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. 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.
       </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;

BEGIN
    -- some computations
    IF stocks &gt; 100000 THEN
        EXIT;  -- causes exit from the BEGIN block
    END IF;
END;</PRE
><P>
       </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN44028"
>38.6.3.3. <TT
CLASS="LITERAL"
>CONTINUE</TT
></A
></H3
><A
NAME="AEN44031"
></A
><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="AEN44050"
>38.6.3.4. <TT
CLASS="LITERAL"
>WHILE</TT
></A
></H3
><A
NAME="AEN44053"
></A
><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"
>38.6.3.5. <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"
>38.6.4. 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
    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 <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 38.10.1</A
> and
     <A
HREF="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING"
>Section 38.10.2</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
> 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.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-ERROR-TRAPPING"
>38.6.5. Trapping Errors</A
></H2
><A
NAME="AEN44143"
></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 <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.
    </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
><P
>     Within an exception handler, the <TT
CLASS="VARNAME"
>SQLSTATE</TT
>
     variable 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 <TT
CLASS="VARNAME"
>SQLERRM</TT
> 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
><B
>Example 38-1. 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>

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