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<chapter id="functions">

<title>Functions and Operators</title>

<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.)

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>

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:

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

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>

<para>

For more information about creating triggers, see

<xref linkend="SQL-CREATETRIGGER">.

<xref linkend="SQL-CREATETRIGGER">.

</para>

</sect1>

</chapter>