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><A
NAME="TRIGGER-DEFINITION"
>36.1. 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 attached to both tables and views.
</P
><P
> On tables, 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.
<TT
CLASS="COMMAND"
>UPDATE</TT
> triggers can moreover be set to fire only if
certain columns are mentioned in the <TT
CLASS="LITERAL"
>SET</TT
> clause of the
<TT
CLASS="COMMAND"
>UPDATE</TT
> 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
> On views, triggers can be defined to execute instead of
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
<TT
CLASS="COMMAND"
>DELETE</TT
> operations. <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers
are fired once for each row that needs to be modified in the view.
It is the responsibility of the
trigger's function to perform the necessary modifications to the
underlying base tables and, where appropriate, return the modified
row as it will appear in the view. Triggers on views can also be defined
to execute once per <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> statement, before or after
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
<TT
CLASS="COMMAND"
>DELETE</TT
> operations.
</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"
>CREATE TRIGGER</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. On views, triggers that fire before or
after may only be defined at statement level, while triggers that fire
instead of an <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
or <TT
CLASS="COMMAND"
>DELETE</TT
> may only be defined at row level.
</P
><P
> Triggers are also classified according to whether they fire
<I
CLASS="FIRSTTERM"
>before</I
>, <I
CLASS="FIRSTTERM"
>after</I
>, or
<I
CLASS="FIRSTTERM"
>instead of</I
> the operation. These are referred to
as <TT
CLASS="LITERAL"
>BEFORE</TT
> triggers, <TT
CLASS="LITERAL"
>AFTER</TT
> triggers, and
<TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers respectively.
Statement-level <TT
CLASS="LITERAL"
>BEFORE</TT
> triggers naturally fire before the
statement starts to do anything, while statement-level <TT
CLASS="LITERAL"
>AFTER</TT
>
triggers fire at the very end of the statement. These types of
triggers may be defined on tables or views. Row-level <TT
CLASS="LITERAL"
>BEFORE</TT
>
triggers fire immediately before a particular row is operated on,
while row-level <TT
CLASS="LITERAL"
>AFTER</TT
> triggers fire at the end of the
statement (but before any statement-level <TT
CLASS="LITERAL"
>AFTER</TT
> triggers).
These types of triggers may only be defined on tables. Row-level
<TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers may only be defined on views, and fire
immediately as each row in the view is identified as needing to be
operated on.
</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,
modification, or deletion 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 <TT
CLASS="LITERAL"
>BEFORE</TT
> 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
> A row-level <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> trigger should either return
<TT
CLASS="SYMBOL"
>NULL</TT
> to indicate that it did not modify any data from
the view's underlying base tables, or it should return the view
row that was passed in (the <TT
CLASS="VARNAME"
>NEW</TT
> row
for <TT
CLASS="COMMAND"
>INSERT</TT
> and <TT
CLASS="COMMAND"
>UPDATE</TT
>
operations, or the <TT
CLASS="VARNAME"
>OLD</TT
> row for
<TT
CLASS="COMMAND"
>DELETE</TT
> operations). A nonnull return value is
used to signal that the trigger performed the necessary data
modifications in the view. This will cause the count of the number
of rows affected by the command to be incremented. For
<TT
CLASS="COMMAND"
>INSERT</TT
> and <TT
CLASS="COMMAND"
>UPDATE</TT
> operations, the trigger
may modify the <TT
CLASS="VARNAME"
>NEW</TT
> row before returning it. This will
change the data returned by
<TT
CLASS="COMMAND"
>INSERT RETURNING</TT
> or <TT
CLASS="COMMAND"
>UPDATE RETURNING</TT
>,
and is useful when the view will not show exactly the same data
that was provided.
</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 <TT
CLASS="LITERAL"
>BEFORE</TT
> and
<TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers, the possibly-modified row returned by
each trigger becomes the input to the next trigger. If any
<TT
CLASS="LITERAL"
>BEFORE</TT
> or <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> trigger returns
<TT
CLASS="SYMBOL"
>NULL</TT
>, the operation is abandoned for that row and subsequent
triggers are not fired (for that row).
</P
><P
> A trigger definition can also specify a Boolean <TT
CLASS="LITERAL"
>WHEN</TT
>
condition, which will be tested to see whether the trigger should
be fired. In row-level triggers the <TT
CLASS="LITERAL"
>WHEN</TT
> condition can
examine the old and/or new values of columns of the row. (Statement-level
triggers can also have <TT
CLASS="LITERAL"
>WHEN</TT
> conditions, although the feature
is not so useful for them.) In a <TT
CLASS="LITERAL"
>BEFORE</TT
> trigger, the
<TT
CLASS="LITERAL"
>WHEN</TT
>
condition is evaluated just before the function is or would be executed,
so using <TT
CLASS="LITERAL"
>WHEN</TT
> is not materially different from testing the
same condition at the beginning of the trigger function. However, in
an <TT
CLASS="LITERAL"
>AFTER</TT
> trigger, the <TT
CLASS="LITERAL"
>WHEN</TT
> 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
<TT
CLASS="LITERAL"
>AFTER</TT
> trigger's
<TT
CLASS="LITERAL"
>WHEN</TT
> 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.
<TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers do not support
<TT
CLASS="LITERAL"
>WHEN</TT
> conditions.
</P
><P
> Typically, row-level <TT
CLASS="LITERAL"
>BEFORE</TT
> triggers are used for checking or
modifying the data that will be inserted or updated. For example,
a <TT
CLASS="LITERAL"
>BEFORE</TT
> 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-level <TT
CLASS="LITERAL"
>AFTER</TT
> 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 <TT
CLASS="LITERAL"
>AFTER</TT
> trigger can be certain it is seeing the final
value of the row, while a <TT
CLASS="LITERAL"
>BEFORE</TT
> trigger cannot; there might
be other <TT
CLASS="LITERAL"
>BEFORE</TT
> triggers firing after it. If you have no
specific reason to make a trigger <TT
CLASS="LITERAL"
>BEFORE</TT
> or
<TT
CLASS="LITERAL"
>AFTER</TT
>, the <TT
CLASS="LITERAL"
>BEFORE</TT
> 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. 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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