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><H1
><A
NAME="SQL-CREATETRIGGER"
></A
>CREATE TRIGGER</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN63342"
></A
><H2
>Name</H2
>CREATE TRIGGER -- define a new trigger</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN63347"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>CREATE TRIGGER <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> { BEFORE | AFTER } { <TT
CLASS="REPLACEABLE"
><I
>event</I
></TT
> [ OR ... ] }
ON <TT
CLASS="REPLACEABLE"
><I
>table</I
></TT
> [ FOR [ EACH ] { ROW | STATEMENT } ]
[ WHEN ( <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> ) ]
EXECUTE PROCEDURE <TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
> ( <TT
CLASS="REPLACEABLE"
><I
>arguments</I
></TT
> )</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN63355"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> creates a new trigger. The
trigger will be associated with the specified table and will
execute the specified function <TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
> when certain events occur.
</P
><P
> The trigger can be specified to fire either before the
operation is attempted on a row (before constraints are checked and
the <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
<TT
CLASS="COMMAND"
>DELETE</TT
> is attempted) or after the operation has
completed (after constraints are checked and the
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, or
<TT
CLASS="COMMAND"
>DELETE</TT
> has completed). If the trigger fires
before the event, the trigger can skip the operation for the
current row, or change the row being inserted (for
<TT
CLASS="COMMAND"
>INSERT</TT
> and <TT
CLASS="COMMAND"
>UPDATE</TT
> operations
only). If the trigger fires after the event, all changes, including
the effects of other triggers, are <SPAN
CLASS="QUOTE"
>"visible"</SPAN
>
to the trigger.
</P
><P
> A trigger that is marked <TT
CLASS="LITERAL"
>FOR EACH ROW</TT
> is called
once for every row that the operation modifies. For example, a
<TT
CLASS="COMMAND"
>DELETE</TT
> that affects 10 rows will cause any
<TT
CLASS="LITERAL"
>ON DELETE</TT
> triggers on the target relation to be
called 10 separate times, once for each deleted row. In contrast, a
trigger that is marked <TT
CLASS="LITERAL"
>FOR EACH STATEMENT</TT
> only
executes once for any given operation, regardless of how many rows
it modifies (in particular, an operation that modifies zero rows
will still result in the execution of any applicable <TT
CLASS="LITERAL"
>FOR
EACH STATEMENT</TT
> triggers).
</P
><P
> In addition, triggers may be defined to fire for a
<TT
CLASS="COMMAND"
>TRUNCATE</TT
>, though only
<TT
CLASS="LITERAL"
>FOR EACH STATEMENT</TT
>.
</P
><P
> Also, a trigger definition can 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 since the condition cannot refer to any values
in the table.
</P
><P
> If multiple triggers of the same kind are defined for the same event,
they will be fired in alphabetical order by name.
</P
><P
> <TT
CLASS="COMMAND"
>SELECT</TT
> does not modify any rows so you cannot
create <TT
CLASS="COMMAND"
>SELECT</TT
> triggers. Rules and views are more
appropriate in such cases.
</P
><P
> Refer to <A
HREF="triggers.html"
>Chapter 36</A
> for more information about triggers.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN63389"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></DT
><DD
><P
> The name to give the new trigger. This must be distinct from
the name of any other trigger for the same table.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>BEFORE</TT
><BR><TT
CLASS="LITERAL"
>AFTER</TT
></DT
><DD
><P
> Determines whether the function is called before or after the
event.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>event</I
></TT
></DT
><DD
><P
> One of <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>TRUNCATE</TT
>;
this specifies the event that will fire the trigger. Multiple
events can be specified using <TT
CLASS="LITERAL"
>OR</TT
>.
</P
><P
> For <TT
CLASS="COMMAND"
>UPDATE</TT
> triggers, it is possible to
specify a list of columns using this syntax:
</P><PRE
CLASS="SYNOPSIS"
>UPDATE OF <TT
CLASS="REPLACEABLE"
><I
>column_name1</I
></TT
> [, <TT
CLASS="REPLACEABLE"
><I
>column_name2</I
></TT
> ... ]</PRE
><P>
The trigger will only fire if at least one of the listed columns
is mentioned as a target of the update.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>table</I
></TT
></DT
><DD
><P
> The name (optionally schema-qualified) of the table the trigger
is for.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>FOR EACH ROW</TT
><BR><TT
CLASS="LITERAL"
>FOR EACH STATEMENT</TT
></DT
><DD
><P
> This specifies whether the trigger procedure should be fired
once for every row affected by the trigger event, or just once
per SQL statement. If neither is specified, <TT
CLASS="LITERAL"
>FOR EACH
STATEMENT</TT
> is the default.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
></DT
><DD
><P
> A Boolean expression that determines whether the trigger function
will actually be executed. If <TT
CLASS="LITERAL"
>WHEN</TT
> is specified, the
function will only be called if the <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> returns <TT
CLASS="LITERAL"
>true</TT
>.
In <TT
CLASS="LITERAL"
>FOR EACH ROW</TT
> triggers, the <TT
CLASS="LITERAL"
>WHEN</TT
>
condition can refer to columns of the old and/or new row values
by writing <TT
CLASS="LITERAL"
>OLD.<TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
></TT
> or
<TT
CLASS="LITERAL"
>NEW.<TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
></TT
> respectively.
Of course, <TT
CLASS="LITERAL"
>INSERT</TT
> triggers cannot refer to <TT
CLASS="LITERAL"
>OLD</TT
>
and <TT
CLASS="LITERAL"
>DELETE</TT
> triggers cannot refer to <TT
CLASS="LITERAL"
>NEW</TT
>.
</P
><P
> Currently, <TT
CLASS="LITERAL"
>WHEN</TT
> expressions cannot contain
subqueries.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
></DT
><DD
><P
> A user-supplied function that is declared as taking no arguments
and returning type <TT
CLASS="LITERAL"
>trigger</TT
>, which is executed when
the trigger fires.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>arguments</I
></TT
></DT
><DD
><P
> An optional comma-separated list of arguments to be provided to
the function when the trigger is executed. The arguments are
literal string constants. Simple names and numeric constants
can be written here, too, but they will all be converted to
strings. Please check the description of the implementation
language of the trigger function to find out how these arguments
can be accessed within the function; it might be different from
normal function arguments.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SQL-CREATETRIGGER-NOTES"
></A
><H2
>Notes</H2
><P
> To create a trigger on a table, the user must have the
<TT
CLASS="LITERAL"
>TRIGGER</TT
> privilege on the table. The user must
also have <TT
CLASS="LITERAL"
>EXECUTE</TT
> privilege on the trigger function.
</P
><P
> Use <A
HREF="sql-droptrigger.html"
>DROP TRIGGER</A
> to remove a trigger.
</P
><P
> A column-specific trigger (<TT
CLASS="LITERAL"
>FOR UPDATE OF
<TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
></TT
>) will fire when any
of its columns are listed as targets in the <TT
CLASS="COMMAND"
>UPDATE</TT
>
command's <TT
CLASS="LITERAL"
>SET</TT
> list. It is possible for a column's value
to change even when the trigger is not fired, because changes made to the
row's contents by <TT
CLASS="LITERAL"
>BEFORE UPDATE</TT
> triggers are not considered.
Conversely, a command such as <TT
CLASS="LITERAL"
>UPDATE ... SET x = x ...</TT
>
will fire a trigger on column <TT
CLASS="LITERAL"
>x</TT
>, even though the column's
value did not change.
</P
><P
> 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. Note in particular
that the <TT
CLASS="LITERAL"
>NEW</TT
> row seen by the condition is the current value,
as possibly modified by earlier triggers. Also, a <TT
CLASS="LITERAL"
>BEFORE</TT
>
trigger's <TT
CLASS="LITERAL"
>WHEN</TT
> condition is not allowed to examine the
system columns of the <TT
CLASS="LITERAL"
>NEW</TT
> row (such as <TT
CLASS="LITERAL"
>oid</TT
>),
because those won't have been set yet.
</P
><P
> 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.
</P
><P
> In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> versions before 7.3, it was
necessary to declare trigger functions as returning the placeholder
type <TT
CLASS="TYPE"
>opaque</TT
>, rather than <TT
CLASS="TYPE"
>trigger</TT
>. To support loading
of old dump files, <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> will accept a function
declared as returning <TT
CLASS="TYPE"
>opaque</TT
>, but it will issue a notice and
change the function's declared return type to <TT
CLASS="TYPE"
>trigger</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SQL-CREATETRIGGER-EXAMPLES"
></A
><H2
>Examples</H2
><P
> Execute the function <CODE
CLASS="FUNCTION"
>check_account_update</CODE
> whenever
a row of the table <TT
CLASS="LITERAL"
>accounts</TT
> is about to be updated:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER check_update
BEFORE UPDATE ON accounts
FOR EACH ROW
EXECUTE PROCEDURE check_account_update();</PRE
><P>
The same, but only execute the function if column <TT
CLASS="LITERAL"
>balance</TT
>
is specified as a target in the <TT
CLASS="COMMAND"
>UPDATE</TT
> command:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER check_update
BEFORE UPDATE OF balance ON accounts
FOR EACH ROW
EXECUTE PROCEDURE check_account_update();</PRE
><P>
This form only executes the function if column <TT
CLASS="LITERAL"
>balance</TT
>
has in fact changed value:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER check_update
BEFORE UPDATE ON accounts
FOR EACH ROW
WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
EXECUTE PROCEDURE check_account_update();</PRE
><P>
Call a function to log updates of <TT
CLASS="LITERAL"
>accounts</TT
>, but only if
something changed:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER log_update
AFTER UPDATE ON accounts
FOR EACH ROW
WHEN (OLD.* IS DISTINCT FROM NEW.*)
EXECUTE PROCEDURE log_account_update();</PRE
><P>
</P
><P
> <A
HREF="trigger-example.html"
>Section 36.4</A
> contains a complete example of a trigger
function written in C.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SQL-CREATETRIGGER-COMPATIBILITY"
></A
><H2
>Compatibility</H2
><P
> The <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> statement in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> implements a subset of the
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> standard. The following functionality is currently missing:
<P
></P
></P><UL
><LI
><P
> SQL allows you to define aliases for the <SPAN
CLASS="QUOTE"
>"old"</SPAN
>
and <SPAN
CLASS="QUOTE"
>"new"</SPAN
> rows or tables for use in the definition
of the triggered action (e.g., <TT
CLASS="LITERAL"
>CREATE TRIGGER ... ON
tablename REFERENCING OLD ROW AS somename NEW ROW AS othername
...</TT
>). Since <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
allows trigger procedures to be written in any number of
user-defined languages, access to the data is handled in a
language-specific way.
</P
></LI
><LI
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> only allows the execution
of a user-defined function for the triggered action. The standard
allows the execution of a number of other SQL commands, such as
<TT
CLASS="COMMAND"
>CREATE TABLE</TT
>, as the triggered action. This
limitation is not hard to work around by creating a user-defined
function that executes the desired commands.
</P
></LI
></UL
><P>
</P
><P
> SQL specifies that multiple triggers should be fired in
time-of-creation order. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uses
name order, which was judged to be more convenient.
</P
><P
> SQL specifies that <TT
CLASS="LITERAL"
>BEFORE DELETE</TT
> triggers on cascaded
deletes fire <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>after</I
></SPAN
> the cascaded <TT
CLASS="LITERAL"
>DELETE</TT
> completes.
The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> behavior is for <TT
CLASS="LITERAL"
>BEFORE
DELETE</TT
> to always fire before the delete action, even a cascading
one. This is considered more consistent. There is also unpredictable
behavior when <TT
CLASS="LITERAL"
>BEFORE</TT
> triggers modify rows or prevent
updates during an update that is caused by a referential action. This can
lead to constraint violations or stored data that does not honor the
referential constraint.
</P
><P
> The ability to specify multiple actions for a single trigger using
<TT
CLASS="LITERAL"
>OR</TT
> is a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extension of
the SQL standard.
</P
><P
> The ability to fire triggers for <TT
CLASS="COMMAND"
>TRUNCATE</TT
> is a
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extension of the SQL standard.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN63546"
></A
><H2
>See Also</H2
><A
HREF="sql-createfunction.html"
>CREATE FUNCTION</A
>, <A
HREF="sql-altertrigger.html"
>ALTER TRIGGER</A
>, <A
HREF="sql-droptrigger.html"
>DROP TRIGGER</A
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