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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/func.sgml | 53 |
1 files changed, 52 insertions, 1 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 7cfb1219889..0aaf4c1b480 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.452 2008/11/03 17:51:12 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.453 2008/11/03 20:17:20 adunstan Exp $ --> <chapter id="functions"> <title>Functions and Operators</title> @@ -12846,4 +12846,55 @@ SELECT (pg_stat_file('filename')).modification; </sect1> + <sect1 id="functions-trigger"> + <title>Trigger Functions</title> + + <indexterm> + <primary>suppress_redundant_updates_trigger</primary> + </indexterm> + + <para> + Currently <productname>PostgreSQL</> provides one built in trigger + function, <function>suppress_redundant_updates_trigger</>, + which will prevent any update + that does not actually change the data in the row from taking place, in + contrast to the normal behaviour which always performs the update + regardless of whether or not the data has changed. (This normal behaviour + makes updates run faster, since no checking is required, and is also + useful in certain cases.) + </para> + + <para> + 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 + <function>suppress_redundant_updates_trigger</>, 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. + </para> + + <para> + The <function>suppress_redundant_updates_trigger</> function can be + added to a table like this: +<programlisting> +CREATE TRIGGER z_min_update +BEFORE UPDATE ON tablename +FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger(); +</programlisting> + 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. + </para> + <para> + For more information about creating triggers, see + <xref linkend="SQL-CREATETRIGGER">. + </para> + </sect1> </chapter> |