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>Chapter 35. Triggers</TD
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><A
NAME="TRIGGER-DEFINITION"
>35.1. Overview of Trigger Behavior</A
></H1
><P
> A trigger is a specification that the database should automatically
execute a particular function whenever a certain type of operation is
performed. Triggers can be defined to execute either before or after any
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
<TT
CLASS="COMMAND"
>DELETE</TT
> operation, either once per modified row,
or once per <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> statement. Triggers can also fire
for <TT
CLASS="COMMAND"
>TRUNCATE</TT
> statements. If a trigger event occurs,
the trigger's function is called at the appropriate time to handle the
event.
</P
><P
> The trigger function must be defined before the trigger itself can be
created. The trigger function must be declared as a
function taking no arguments and returning type <TT
CLASS="LITERAL"
>trigger</TT
>.
(The trigger function receives its input through a specially-passed
<TT
CLASS="STRUCTNAME"
>TriggerData</TT
> structure, not in the form of ordinary function
arguments.)
</P
><P
> Once a suitable trigger function has been created, the trigger is
established with
<A
HREF="sql-createtrigger.html"
><I
>CREATE TRIGGER</I
></A
>.
The same trigger function can be used for multiple triggers.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> offers both <I
CLASS="FIRSTTERM"
>per-row</I
>
triggers and <I
CLASS="FIRSTTERM"
>per-statement</I
> triggers. With a per-row
trigger, the trigger function
is invoked once for each row that is affected by the statement
that fired the trigger. In contrast, a per-statement trigger is
invoked only once when an appropriate statement is executed,
regardless of the number of rows affected by that statement. In
particular, a statement that affects zero rows will still result
in the execution of any applicable per-statement triggers. These
two types of triggers are sometimes called <I
CLASS="FIRSTTERM"
>row-level</I
>
triggers and <I
CLASS="FIRSTTERM"
>statement-level</I
> triggers,
respectively. Triggers on <TT
CLASS="COMMAND"
>TRUNCATE</TT
> may only be
defined at statement-level.
</P
><P
> Triggers are also classified as <I
CLASS="FIRSTTERM"
>before</I
> triggers and
<I
CLASS="FIRSTTERM"
>after</I
> triggers.
Statement-level before triggers naturally fire before the
statement starts to do anything, while statement-level after
triggers fire at the very end of the statement. Row-level before
triggers fire immediately before a particular row is operated on,
while row-level after triggers fire at the end of the statement
(but before any statement-level after triggers).
</P
><P
> Trigger functions invoked by per-statement triggers should always
return <TT
CLASS="SYMBOL"
>NULL</TT
>. Trigger functions invoked by per-row
triggers can return a table row (a value of
type <TT
CLASS="STRUCTNAME"
>HeapTuple</TT
>) to the calling executor,
if they choose. A row-level trigger fired before an operation has
the following choices:
<P
></P
></P><UL
><LI
><P
> It can return <TT
CLASS="SYMBOL"
>NULL</TT
> to skip the operation for the
current row. This instructs the executor to not perform the
row-level operation that invoked the trigger (the insertion or
modification of a particular table row).
</P
></LI
><LI
><P
> For row-level <TT
CLASS="COMMAND"
>INSERT</TT
>
and <TT
CLASS="COMMAND"
>UPDATE</TT
> triggers only, the returned row
becomes the row that will be inserted or will replace the row
being updated. This allows the trigger function to modify the
row being inserted or updated.
</P
></LI
></UL
><P>
A row-level before trigger that does not intend to cause either of
these behaviors must be careful to return as its result the same
row that was passed in (that is, the <TT
CLASS="VARNAME"
>NEW</TT
> row
for <TT
CLASS="COMMAND"
>INSERT</TT
> and <TT
CLASS="COMMAND"
>UPDATE</TT
>
triggers, the <TT
CLASS="VARNAME"
>OLD</TT
> row for
<TT
CLASS="COMMAND"
>DELETE</TT
> triggers).
</P
><P
> The return value is ignored for row-level triggers fired after an
operation, and so they can return <TT
CLASS="SYMBOL"
>NULL</TT
>.
</P
><P
> If more than one trigger is defined for the same event on the same
relation, the triggers will be fired in alphabetical order by
trigger name. In the case of before triggers, the
possibly-modified row returned by each trigger becomes the input
to the next trigger. If any before trigger returns
<TT
CLASS="SYMBOL"
>NULL</TT
>, the operation is abandoned for that row and subsequent
triggers are not fired.
</P
><P
> Typically, row before triggers are used for checking or
modifying the data that will be inserted or updated. For example,
a before trigger might be used to insert the current time into a
<TT
CLASS="TYPE"
>timestamp</TT
> column, or to check that two elements of the row are
consistent. Row after triggers are most sensibly
used to propagate the updates to other tables, or make consistency
checks against other tables. The reason for this division of labor is
that an after trigger can be certain it is seeing the final value of the
row, while a before trigger cannot; there might be other before triggers
firing after it. If you have no specific reason to make a trigger before
or after, the before case is more efficient, since the information about
the operation doesn't have to be saved until end of statement.
</P
><P
> If a trigger function executes SQL commands then these
commands might fire triggers again. This is known as cascading
triggers. There is no direct limitation on the number of cascade
levels. It is possible for cascades to cause a recursive invocation
of the same trigger; for example, an <TT
CLASS="COMMAND"
>INSERT</TT
>
trigger might execute a command that inserts an additional row
into the same table, causing the <TT
CLASS="COMMAND"
>INSERT</TT
> trigger
to be fired again. It is the trigger programmer's responsibility
to avoid infinite recursion in such scenarios.
</P
><P
> When a trigger is being defined, arguments can be specified for
it.<A
NAME="AEN46007"
></A
> The purpose of including arguments in the
trigger definition is to allow different triggers with similar
requirements to call the same function. As an example, there
could be a generalized trigger function that takes as its
arguments two column names and puts the current user in one and
the current time stamp in the other. Properly written, this
trigger function would be independent of the specific table it is
triggering on. So the same function could be used for
<TT
CLASS="COMMAND"
>INSERT</TT
> events on any table with suitable
columns, to automatically track creation of records in a
transaction table for example. It could also be used to track
last-update events if defined as an <TT
CLASS="COMMAND"
>UPDATE</TT
>
trigger.
</P
><P
> Each programming language that supports triggers has its own method
for making the trigger input data available to the trigger function.
This input data includes the type of trigger event (e.g.,
<TT
CLASS="COMMAND"
>INSERT</TT
> or <TT
CLASS="COMMAND"
>UPDATE</TT
>) as well as any
arguments that were listed in <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
>.
For a row-level trigger, the input data also includes the
<TT
CLASS="VARNAME"
>NEW</TT
> row for <TT
CLASS="COMMAND"
>INSERT</TT
> and
<TT
CLASS="COMMAND"
>UPDATE</TT
> triggers, and/or the <TT
CLASS="VARNAME"
>OLD</TT
> row
for <TT
CLASS="COMMAND"
>UPDATE</TT
> and <TT
CLASS="COMMAND"
>DELETE</TT
> triggers.
Statement-level triggers do not currently have any way to examine the
individual row(s) modified by the statement.
</P
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