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Add a WHEN clause to CREATE TRIGGER, allowing a boolean expression to be

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checked to determine whether the trigger should be fired.

For BEFORE triggers this is mostly a matter of spec compliance; but for AFTER
triggers it can provide a noticeable performance improvement, since queuing of
a deferred trigger event and re-fetching of the row(s) at end of statement can
be short-circuited if the trigger does not need to be fired.

Takahiro Itagaki, reviewed by KaiGai Kohei.
This commit is contained in:
Tom Lane
2009-11-20 20:38:12 +00:00
parent 201a45c4fa
commit 7fc0f06221
27 changed files with 783 additions and 100 deletions
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22
doc/src/sgml/trigger.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.59 2009/10/14 22:14:21 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.60 2009/11/20 20:38:09 tgl Exp $ -->
<chapter id="triggers">
<title>Triggers</title>
@@ -140,6 +140,25 @@
triggers are not fired.
</para>
<para>
A trigger definition can also specify a boolean <literal>WHEN</>
condition, which will be tested to see whether the trigger should
be fired. In row-level triggers the <literal>WHEN</> condition can
examine the old and/or new values of columns of the row. (Statement-level
triggers can also have <literal>WHEN</> conditions, although the feature
is not so useful for them.) In a before trigger, the <literal>WHEN</>
condition is evaluated just before the function is or would be executed,
so using <literal>WHEN</> is not materially different from testing the
same condition at the beginning of the trigger function. However, in
an after trigger, the <literal>WHEN</> condition is evaluated just after
the row update occurs, and it determines whether an event is queued to
fire the trigger at the end of statement. So when an after trigger's
<literal>WHEN</> condition does not return true, it is not necessary
to queue an event nor to re-fetch the row at end of statement. This
can result in significant speedups in statements that modify many
rows, if the trigger only needs to be fired for a few of the rows.
</para>
<para>
Typically, row before triggers are used for checking or
modifying the data that will be inserted or updated. For example,
@@ -497,6 +516,7 @@ typedef struct Trigger
int16 tgnattr;
int16 *tgattr;
char **tgargs;
char *tgqual;
} Trigger;
</programlisting>