| Server IP : 103.119.228.120 / Your IP : 3.142.136.210 Web Server : Apache System : Linux v8.techscape8.com 3.10.0-1160.119.1.el7.tuxcare.els2.x86_64 #1 SMP Mon Jul 15 12:09:18 UTC 2024 x86_64 User : nobody ( 99) PHP Version : 5.6.40 Disable Function : shell_exec,symlink,system,exec,proc_get_status,proc_nice,proc_terminate,define_syslog_variables,syslog,openlog,closelog,escapeshellcmd,passthru,ocinum cols,ini_alter,leak,listen,chgrp,apache_note,apache_setenv,debugger_on,debugger_off,ftp_exec,dl,dll,myshellexec,proc_open,socket_bind,proc_close,escapeshellarg,parse_ini_filepopen,fpassthru,exec,passthru,escapeshellarg,escapeshellcmd,proc_close,proc_open,ini_alter,popen,show_source,proc_nice,proc_terminate,proc_get_status,proc_close,pfsockopen,leak,apache_child_terminate,posix_kill,posix_mkfifo,posix_setpgid,posix_setsid,posix_setuid,dl,symlink,shell_exec,system,dl,passthru,escapeshellarg,escapeshellcmd,myshellexec,c99_buff_prepare,c99_sess_put,fpassthru,getdisfunc,fx29exec,fx29exec2,is_windows,disp_freespace,fx29sh_getupdate,fx29_buff_prepare,fx29_sess_put,fx29shexit,fx29fsearch,fx29ftpbrutecheck,fx29sh_tools,fx29sh_about,milw0rm,imagez,sh_name,myshellexec,checkproxyhost,dosyayicek,c99_buff_prepare,c99_sess_put,c99getsource,c99sh_getupdate,c99fsearch,c99shexit,view_perms,posix_getpwuid,posix_getgrgid,posix_kill,parse_perms,parsesort,view_perms_color,set_encoder_input,ls_setcheckboxall,ls_reverse_all,rsg_read,rsg_glob,selfURL,dispsecinfo,unix2DosTime,addFile,system,get_users,view_size,DirFiles,DirFilesWide,DirPrintHTMLHeaders,GetFilesTotal,GetTitles,GetTimeTotal,GetMatchesCount,GetFileMatchesCount,GetResultFiles,fs_copy_dir,fs_copy_obj,fs_move_dir,fs_move_obj,fs_rmdir,SearchText,getmicrotime MySQL : ON | cURL : ON | WGET : ON | Perl : ON | Python : ON | Sudo : ON | Pkexec : ON Directory : /usr/local/ssl/share/doc/postgresql-9.2.24/html/ |
Upload File : |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>CREATE TRIGGER</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.2.24 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="SQL Commands"
HREF="sql-commands.html"><LINK
REL="PREVIOUS"
TITLE="CREATE TEXT SEARCH TEMPLATE"
HREF="sql-createtstemplate.html"><LINK
REL="NEXT"
TITLE="CREATE TYPE"
HREF="sql-createtype.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2017-11-06T22:43:11"></HEAD
><BODY
CLASS="REFENTRY"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.2.24 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="CREATE TEXT SEARCH TEMPLATE"
HREF="sql-createtstemplate.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="sql-commands.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="CREATE TYPE"
HREF="sql-createtype.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><H1
><A
NAME="SQL-CREATETRIGGER"
></A
>CREATE TRIGGER</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN71782"
></A
><H2
>Name</H2
>CREATE TRIGGER -- define a new trigger</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN71787"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>CREATE [ CONSTRAINT ] TRIGGER <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> { BEFORE | AFTER | INSTEAD OF } { <TT
CLASS="REPLACEABLE"
><I
>event</I
></TT
> [ OR ... ] }
ON <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
>
[ FROM <TT
CLASS="REPLACEABLE"
><I
>referenced_table_name</I
></TT
> ]
[ NOT DEFERRABLE | [ DEFERRABLE ] { INITIALLY IMMEDIATE | INITIALLY DEFERRED } ]
[ 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
> )
<SPAN
CLASS="phrase"
><SPAN
CLASS="PHRASE"
>where <TT
CLASS="REPLACEABLE"
><I
>event</I
></TT
> can be one of:</SPAN
></SPAN
>
INSERT
UPDATE [ OF <TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
> [, ... ] ]
DELETE
TRUNCATE</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN71799"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> creates a new trigger. The
trigger will be associated with the specified table or view 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 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); or instead of the operation
(in the case of inserts, updates or deletes on a view).
If the trigger fires before or instead of 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
> Triggers that are specified to fire <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> the trigger
event must be marked <TT
CLASS="LITERAL"
>FOR EACH ROW</TT
>, and can only be defined
on views. <TT
CLASS="LITERAL"
>BEFORE</TT
> and <TT
CLASS="LITERAL"
>AFTER</TT
> triggers on a view
must be marked as <TT
CLASS="LITERAL"
>FOR EACH STATEMENT</TT
>.
</P
><P
> In addition, triggers may be defined to fire for
<TT
CLASS="COMMAND"
>TRUNCATE</TT
>, though only
<TT
CLASS="LITERAL"
>FOR EACH STATEMENT</TT
>.
</P
><P
> The following table summarizes which types of triggers may be used on
tables and views:
</P
><DIV
CLASS="INFORMALTABLE"
><P
></P
><A
NAME="SUPPORTED-TRIGGER-TYPES"
></A
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>When</TH
><TH
>Event</TH
><TH
>Row-level</TH
><TH
>Statement-level</TH
></TR
></THEAD
><TBODY
><TR
><TD
ROWSPAN="2"
ALIGN="CENTER"
><TT
CLASS="LITERAL"
>BEFORE</TT
></TD
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>INSERT</TT
>/<TT
CLASS="COMMAND"
>UPDATE</TT
>/<TT
CLASS="COMMAND"
>DELETE</TT
></TD
><TD
ALIGN="CENTER"
>Tables</TD
><TD
ALIGN="CENTER"
>Tables and views</TD
></TR
><TR
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>TRUNCATE</TT
></TD
><TD
ALIGN="CENTER"
>—</TD
><TD
ALIGN="CENTER"
>Tables</TD
></TR
><TR
><TD
ROWSPAN="2"
ALIGN="CENTER"
><TT
CLASS="LITERAL"
>AFTER</TT
></TD
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>INSERT</TT
>/<TT
CLASS="COMMAND"
>UPDATE</TT
>/<TT
CLASS="COMMAND"
>DELETE</TT
></TD
><TD
ALIGN="CENTER"
>Tables</TD
><TD
ALIGN="CENTER"
>Tables and views</TD
></TR
><TR
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>TRUNCATE</TT
></TD
><TD
ALIGN="CENTER"
>—</TD
><TD
ALIGN="CENTER"
>Tables</TD
></TR
><TR
><TD
ROWSPAN="2"
ALIGN="CENTER"
><TT
CLASS="LITERAL"
>INSTEAD OF</TT
></TD
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>INSERT</TT
>/<TT
CLASS="COMMAND"
>UPDATE</TT
>/<TT
CLASS="COMMAND"
>DELETE</TT
></TD
><TD
ALIGN="CENTER"
>Views</TD
><TD
ALIGN="CENTER"
>—</TD
></TR
><TR
><TD
ALIGN="CENTER"
><TT
CLASS="COMMAND"
>TRUNCATE</TT
></TD
><TD
ALIGN="CENTER"
>—</TD
><TD
ALIGN="CENTER"
>—</TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
><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
> When the <TT
CLASS="LITERAL"
>CONSTRAINT</TT
> option is specified, this command creates a
<I
CLASS="FIRSTTERM"
>constraint trigger</I
>. This is the same as a regular trigger
except that the timing of the trigger firing can be adjusted using
<A
HREF="sql-set-constraints.html"
>SET CONSTRAINTS</A
>.
Constraint triggers must be <TT
CLASS="LITERAL"
>AFTER ROW</TT
> triggers. They can
be fired either at the end of the statement causing the triggering event,
or at the end of the containing transaction; in the latter case they are
said to be <I
CLASS="FIRSTTERM"
>deferred</I
>. A pending deferred-trigger firing can
also be forced to happen immediately by using <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
>.
Constraint triggers are expected to raise an exception when the constraints
they implement are violated.
</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="AEN71898"
></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.
The name cannot be schema-qualified — the trigger inherits the
schema of its table. For a constraint trigger, this is also the name to
use when modifying the trigger's behavior using
<TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>BEFORE</TT
><BR><TT
CLASS="LITERAL"
>AFTER</TT
><BR><TT
CLASS="LITERAL"
>INSTEAD OF</TT
></DT
><DD
><P
> Determines whether the function is called before, after, or instead of
the event. A constraint trigger can only be specified as
<TT
CLASS="LITERAL"
>AFTER</TT
>.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>event</I
></TT
></DT
><DD
><P
> One of <TT
CLASS="LITERAL"
>INSERT</TT
>, <TT
CLASS="LITERAL"
>UPDATE</TT
>,
<TT
CLASS="LITERAL"
>DELETE</TT
>, or <TT
CLASS="LITERAL"
>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="LITERAL"
>UPDATE</TT
> events, 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 <TT
CLASS="COMMAND"
>UPDATE</TT
> command.
</P
><P
><TT
CLASS="LITERAL"
>INSTEAD OF UPDATE</TT
> events do not support lists of columns.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
></DT
><DD
><P
> The name (optionally schema-qualified) of the table or view the trigger
is for.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>referenced_table_name</I
></TT
></DT
><DD
><P
> The (possibly schema-qualified) name of another table referenced by the
constraint. This option is used for foreign-key constraints and is not
recommended for general use. This can only be specified for
constraint triggers.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DEFERRABLE</TT
><BR><TT
CLASS="LITERAL"
>NOT DEFERRABLE</TT
><BR><TT
CLASS="LITERAL"
>INITIALLY IMMEDIATE</TT
><BR><TT
CLASS="LITERAL"
>INITIALLY DEFERRED</TT
></DT
><DD
><P
> The default timing of the trigger.
See the <A
HREF="sql-createtable.html"
>CREATE TABLE</A
> documentation for details of
these constraint options. This can only be specified for constraint
triggers.
</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. Constraint triggers can only
be specified <TT
CLASS="LITERAL"
>FOR EACH ROW</TT
>.
</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
><TT
CLASS="LITERAL"
>INSTEAD OF</TT
> triggers do not support <TT
CLASS="LITERAL"
>WHEN</TT
>
conditions.
</P
><P
> Currently, <TT
CLASS="LITERAL"
>WHEN</TT
> expressions cannot contain
subqueries.
</P
><P
> Note that for constraint triggers, evaluation of the <TT
CLASS="LITERAL"
>WHEN</TT
>
condition is not deferred, but occurs immediately after the row update
operation is performed. If the condition does not evaluate to true then
the trigger is not queued for deferred execution.
</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 (one defined using the <TT
CLASS="LITERAL"
>UPDATE OF
<TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
></TT
> syntax) 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>
Execute the function <CODE
CLASS="FUNCTION"
>view_insert_row</CODE
> for each row to insert
rows into the tables underlying a view:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER view_insert
INSTEAD OF INSERT ON my_view
FOR EACH ROW
EXECUTE PROCEDURE view_insert_row();</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 nonstandard
behavior if <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, as is the
ability to define statement-level triggers on views.
</P
><P
> <TT
CLASS="COMMAND"
>CREATE CONSTRAINT TRIGGER</TT
> is a
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extension of the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
standard.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72091"
></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
>, <A
HREF="sql-set-constraints.html"
>SET CONSTRAINTS</A
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="sql-createtstemplate.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="sql-createtype.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>CREATE TEXT SEARCH TEMPLATE</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="sql-commands.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>CREATE TYPE</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>