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>36.2. Visibility of Data Changes</A
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><P
>    If you execute SQL commands in your trigger function, and these
    commands access the table that the trigger is for, then
    you need to be aware of the data visibility rules, because they determine
    whether these SQL commands will see the data change that the trigger
    is fired for.  Briefly:

    <P
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>       Statement-level triggers follow simple visibility rules: none of
       the changes made by a statement are visible to statement-level
       triggers that are invoked before the statement, whereas all
       modifications are visible to statement-level <TT
CLASS="LITERAL"
>AFTER</TT
>
       triggers.
      </P
></LI
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>       The data change (insertion, update, or deletion) causing the
       trigger to fire is naturally <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> visible
       to SQL commands executed in a row-level <TT
CLASS="LITERAL"
>BEFORE</TT
> trigger,
       because it hasn't happened yet.
      </P
></LI
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>       However, SQL commands executed in a row-level <TT
CLASS="LITERAL"
>BEFORE</TT
>
       trigger <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>will</I
></SPAN
> see the effects of data
       changes for rows previously processed in the same outer
       command.  This requires caution, since the ordering of these
       change events is not in general predictable; a SQL command that
       affects multiple rows can visit the rows in any order.
      </P
></LI
><LI
><P
>       Similarly, a row-level <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> trigger will see the
       effects of data changes made by previous firings of <TT
CLASS="LITERAL"
>INSTEAD
       OF</TT
> triggers in the same outer command.
      </P
></LI
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><P
>       When a row-level <TT
CLASS="LITERAL"
>AFTER</TT
> trigger is fired, all data
       changes made
       by the outer command are already complete, and are visible to
       the invoked trigger function.
      </P
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   </P
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>    If your trigger function is written in any of the standard procedural
    languages, then the above statements apply only if the function is
    declared <TT
CLASS="LITERAL"
>VOLATILE</TT
>.  Functions that are declared
    <TT
CLASS="LITERAL"
>STABLE</TT
> or <TT
CLASS="LITERAL"
>IMMUTABLE</TT
> will not see changes made by
    the calling command in any case.
   </P
><P
>    Further information about data visibility rules can be found in
    <A
HREF="spi-visibility.html"
>Section 43.4</A
>.  The example in <A
HREF="trigger-example.html"
>Section 36.4</A
> contains a demonstration of these rules.
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