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><A
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>35.2. Visibility of Data Changes</A
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>    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:

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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 after triggers.
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>       The data change (insertion, update, or deletion) causing the
       trigger to fire is naturally <SPAN
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>not</I
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> visible
       to SQL commands executed in a row-level before trigger, because
       it hasn't happened yet.
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>       However, SQL commands executed in a row-level before
       trigger <SPAN
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>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
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>       When a row-level after 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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>    Further information about data visibility rules can be found in
    <A
HREF="spi-visibility.html"
>Section 42.4</A
>.  The example in <A
HREF="trigger-example.html"
>Section 35.4</A
> contains a demonstration of these rules.
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