文件操作 - functions-trigger.html

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>9.27. Trigger Functions</A
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>      Currently <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides one built in trigger
      function, <CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
>,
      which will prevent any update
      that does not actually change the data in the row from taking place, in
      contrast to the normal behavior which always performs the update
      regardless of whether or not the data has changed. (This normal behavior
      makes updates run faster, since no checking is required, and is also
      useful in certain cases.)
    </P
><P
>      Ideally, you should normally avoid running updates that don't actually
      change the data in the record. Redundant updates can cost considerable
      unnecessary time, especially if there are lots of indexes to alter,
      and space in dead rows that will eventually have to be vacuumed.
      However, detecting such situations in client code is not
      always easy, or even possible, and writing expressions to detect
      them can be error-prone. An alternative is to use
      <CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
>, which will skip
      updates that don't change the data. You should use this with care,
      however. The trigger takes a small but non-trivial time for each record,
      so if most of the records affected by an update are actually changed,
      use of this trigger will actually make the update run slower.
    </P
><P
>      The <CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
> function can be
      added to a table like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER z_min_update
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();</PRE
><P>
      In most cases, you would want to fire this trigger last for each row.
      Bearing in mind that triggers fire in name order, you would then
      choose a trigger name that comes after the name of any other trigger
      you might have on the table.
    </P
><P
>       For more information about creating triggers, see
        <A
HREF="sql-createtrigger.html"
>CREATE TRIGGER</A
>.
    </P
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文件操作 - functions-trigger.html

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