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Postgres documentation

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</para>

<para>

The trigger can be specified to fire either before BEFORE the

operation is attempted on a tuple (before constraints

are checked and the <command>INSERT</command>, <command>UPDATE</command> or

<command>DELETE</command> is attempted) or

AFTER the operation has been attempted (e.g., after constraints

are checked and the <command>INSERT</command>,

<command>UPDATE</command> or <command>DELETE</command> has

completed). If the

trigger fires before the event, the trigger may

skip the operation for the current tuple, or change the tuple

being inserted (for <command>INSERT</command> and

<command>UPDATE</command> operations only). If

the trigger fires after the event, all changes, including the

last insertion, update, or deletion, are <quote>visible</quote> to the trigger.

The trigger can be specified to fire either before BEFORE the

operation is attempted on a tuple (before constraints are checked and

the <command>INSERT</command>, <command>UPDATE</command> or

<command>DELETE</command> is attempted) or AFTER the operation has

been attempted (e.g., after constraints are checked and the

<command>INSERT</command>, <command>UPDATE</command> or

<command>DELETE</command> has completed). If the trigger fires before

the event, the trigger may skip the operation for the current tuple,

or change the tuple being inserted (for <command>INSERT</command> and

<command>UPDATE</command> operations only). If the trigger fires

after the event, all changes, including the last insertion, update,

or deletion, are <quote>visible</quote> to the trigger.

</para>

<para>

<command>SELECT</command> does not modify any rows so you can not

create <command>SELECT</command> triggers.

</para>

<para>