Sophie: postgresql8.2-docs-8.2.14-1mdv2010.0 i586

postgresql8.2-docs-8.2.14-1mdv2010.0.i586.rpm

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>PostgreSQL 8.2.14 Documentation</TH
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>Chapter 37. <SPAN
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> - <ACRONYM
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>SQL</ACRONYM
> Procedural Language</TD
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><H1
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><A
NAME="PLPGSQL-EXPRESSIONS"
>37.5. Expressions</A
></H1
><P
>     All expressions used in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
     statements are processed using the server's regular
     <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> executor.  In effect, a query like
</P><PRE
CLASS="SYNOPSIS"
>SELECT <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
></PRE
><P>
     is executed using the <ACRONYM
CLASS="ACRONYM"
>SPI</ACRONYM
> manager. Before evaluation,
     occurrences of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> variable
     identifiers are replaced by parameters, and the actual values from
     the variables are passed to the executor in the parameter array.
     This allows the query plan for the <TT
CLASS="COMMAND"
>SELECT</TT
> to
     be prepared just once and then reused for subsequent
     evaluations.
    </P
><P
>     The evaluation done by the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
     main parser has some side
     effects on the interpretation of constant values. In detail there
     is a difference between what these two functions do:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION logfunc1(logtxt text) RETURNS timestamp AS $$
    BEGIN
        INSERT INTO logtable VALUES (logtxt, 'now');
        RETURN 'now';
    END;
$$ LANGUAGE plpgsql;</PRE
><P>

     and

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION logfunc2(logtxt text) RETURNS timestamp AS $$
    DECLARE
        curtime timestamp;
    BEGIN
        curtime := 'now';
        INSERT INTO logtable VALUES (logtxt, curtime);
        RETURN curtime;
    END;
$$ LANGUAGE plpgsql;</PRE
><P>
    </P
><P
>     In the case of <CODE
CLASS="FUNCTION"
>logfunc1</CODE
>, the
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> main parser knows when
     preparing the plan for the <TT
CLASS="COMMAND"
>INSERT</TT
> that the
     string <TT
CLASS="LITERAL"
>'now'</TT
> should be interpreted as
     <TT
CLASS="TYPE"
>timestamp</TT
> because the target column of
     <CODE
CLASS="CLASSNAME"
>logtable</CODE
> is of that type. Thus,
     <TT
CLASS="LITERAL"
>'now'</TT
> will be converted to a constant when the
     <TT
CLASS="COMMAND"
>INSERT</TT
> is planned, and then used in all
     invocations of <CODE
CLASS="FUNCTION"
>logfunc1</CODE
> during the lifetime
     of the session. Needless to say, this isn't what the programmer
     wanted.
    </P
><P
>     In the case of <CODE
CLASS="FUNCTION"
>logfunc2</CODE
>, the 
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> main parser does not know
     what type <TT
CLASS="LITERAL"
>'now'</TT
> should become and therefore 
     it returns a data value of type <TT
CLASS="TYPE"
>text</TT
> containing the string 
     <TT
CLASS="LITERAL"
>now</TT
>. During the ensuing assignment
     to the local variable <TT
CLASS="VARNAME"
>curtime</TT
>, the
     <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> interpreter casts this
     string to the <TT
CLASS="TYPE"
>timestamp</TT
> type by calling the
     <CODE
CLASS="FUNCTION"
>text_out</CODE
> and <CODE
CLASS="FUNCTION"
>timestamp_in</CODE
>
     functions for the conversion.  So, the computed time stamp is updated
     on each execution as the programmer expects.
    </P
><P
>     The mutable nature of record variables presents a problem in this
     connection.  When fields of a record variable are used in
     expressions or statements, the data types of the fields must not
     change between calls of one and the same expression, since the
     expression will be planned using the data type that is present
     when the expression is first reached.  Keep this in mind when
     writing trigger procedures that handle events for more than one
     table.  (<TT
CLASS="COMMAND"
>EXECUTE</TT
> can be used to get around
     this problem when necessary.)
    </P
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