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><H1
><A
NAME="SQL-CREATETRIGGER"
></A
>CREATE TRIGGER</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN59917"
></A
><H2
>Name</H2
>CREATE TRIGGER -- define a new trigger</DIV
><A
NAME="AEN59920"
></A
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN59922"
></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 } ]
EXECUTE PROCEDURE <TT
CLASS="REPLACEABLE"
><I
>funcname</I
></TT
> ( <TT
CLASS="REPLACEABLE"
><I
>arguments</I
></TT
> )</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN59929"
></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
>funcname</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 last insertion, update, or deletion, 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
> 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 35</A
> for more information about triggers.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN59959"
></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
></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
>funcname</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 about how the trigger arguments
are accessible 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"
><I
>DROP TRIGGER</I
></A
> to remove a trigger.
</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="R1-SQL-CREATETRIGGER-2"
></A
><H2
>Examples</H2
><P
> <A
HREF="trigger-example.html"
>Section 35.4</A
> contains a complete example.
</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 triggers to fire on updates to specific columns
(e.g., <TT
CLASS="LITERAL"
>AFTER UPDATE OF col1, col2</TT
>).
</P
></LI
><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 that are later
to be modified by referential actions. 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="AEN60061"
></A
><H2
>See Also</H2
><A
HREF="sql-createfunction.html"
><I
>CREATE FUNCTION</I
></A
>, <A
HREF="sql-altertrigger.html"
><I
>ALTER TRIGGER</I
></A
>, <A
HREF="sql-droptrigger.html"
><I
>DROP TRIGGER</I
></A
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