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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Trigger Procedures in PL/Tcl</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="PL/Tcl - Tcl Procedural Language" HREF="pltcl.html"><LINK REL="PREVIOUS" TITLE="Database Access from PL/Tcl" HREF="pltcl-dbaccess.html"><LINK REL="NEXT" TITLE="Modules and the unknown Command" HREF="pltcl-unknown.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="SECT1" ><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="Database Access from PL/Tcl" HREF="pltcl-dbaccess.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="pltcl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 40. PL/Tcl - Tcl Procedural Language</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Modules and the unknown Command" HREF="pltcl-unknown.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="PLTCL-TRIGGER" >40.6. Trigger Procedures in PL/Tcl</A ></H1 ><P > Trigger procedures can be written in PL/Tcl. <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > requires that a procedure that is to be called as a trigger must be declared as a function with no arguments and a return type of <TT CLASS="LITERAL" >trigger</TT >. </P ><P > The information from the trigger manager is passed to the procedure body in the following variables: <P ></P ></P><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="VARNAME" >$TG_name</TT ></DT ><DD ><P > The name of the trigger from the <TT CLASS="COMMAND" >CREATE TRIGGER</TT > statement. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_relid</TT ></DT ><DD ><P > The object ID of the table that caused the trigger procedure to be invoked. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_table_name</TT ></DT ><DD ><P > The name of the table that caused the trigger procedure to be invoked. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_table_schema</TT ></DT ><DD ><P > The schema of the table that caused the trigger procedure to be invoked. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_relatts</TT ></DT ><DD ><P > A Tcl list of the table column names, prefixed with an empty list element. So looking up a column name in the list with <SPAN CLASS="APPLICATION" >Tcl</SPAN >'s <CODE CLASS="FUNCTION" >lsearch</CODE > command returns the element's number starting with 1 for the first column, the same way the columns are customarily numbered in <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN >. (Empty list elements also appear in the positions of columns that have been dropped, so that the attribute numbering is correct for columns to their right.) </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_when</TT ></DT ><DD ><P > The string <TT CLASS="LITERAL" >BEFORE</TT >, <TT CLASS="LITERAL" >AFTER</TT >, or <TT CLASS="LITERAL" >INSTEAD OF</TT >, depending on the type of trigger event. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_level</TT ></DT ><DD ><P > The string <TT CLASS="LITERAL" >ROW</TT > or <TT CLASS="LITERAL" >STATEMENT</TT > depending on the type of trigger event. </P ></DD ><DT ><TT CLASS="VARNAME" >$TG_op</TT ></DT ><DD ><P > The string <TT CLASS="LITERAL" >INSERT</TT >, <TT CLASS="LITERAL" >UPDATE</TT >, <TT CLASS="LITERAL" >DELETE</TT >, or <TT CLASS="LITERAL" >TRUNCATE</TT > depending on the type of trigger event. </P ></DD ><DT ><TT CLASS="VARNAME" >$NEW</TT ></DT ><DD ><P > An associative array containing the values of the new table row for <TT CLASS="COMMAND" >INSERT</TT > or <TT CLASS="COMMAND" >UPDATE</TT > actions, or empty for <TT CLASS="COMMAND" >DELETE</TT >. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers. </P ></DD ><DT ><TT CLASS="VARNAME" >$OLD</TT ></DT ><DD ><P > An associative array containing the values of the old table row for <TT CLASS="COMMAND" >UPDATE</TT > or <TT CLASS="COMMAND" >DELETE</TT > actions, or empty for <TT CLASS="COMMAND" >INSERT</TT >. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers. </P ></DD ><DT ><TT CLASS="VARNAME" >$args</TT ></DT ><DD ><P > A Tcl list of the arguments to the procedure as given in the <TT CLASS="COMMAND" >CREATE TRIGGER</TT > statement. These arguments are also accessible as <TT CLASS="LITERAL" >$1</TT > ... <TT CLASS="LITERAL" >$<TT CLASS="REPLACEABLE" ><I >n</I ></TT ></TT > in the procedure body. </P ></DD ></DL ></DIV ><P> </P ><P > The return value from a trigger procedure can be one of the strings <TT CLASS="LITERAL" >OK</TT > or <TT CLASS="LITERAL" >SKIP</TT >, or a list of column name/value pairs. If the return value is <TT CLASS="LITERAL" >OK</TT >, the operation (<TT CLASS="COMMAND" >INSERT</TT >/<TT CLASS="COMMAND" >UPDATE</TT >/<TT CLASS="COMMAND" >DELETE</TT >) that fired the trigger will proceed normally. <TT CLASS="LITERAL" >SKIP</TT > tells the trigger manager to silently suppress the operation for this row. If a list is returned, it tells PL/Tcl to return a modified row to the trigger manager; the contents of the modified row are specified by the column names and values in the list. Any columns not mentioned in the list are set to null. Returning a modified row is only meaningful for row-level <TT CLASS="LITERAL" >BEFORE</TT > <TT CLASS="COMMAND" >INSERT</TT > or <TT CLASS="COMMAND" >UPDATE</TT > triggers, for which the modified row will be inserted instead of the one given in <TT CLASS="VARNAME" >$NEW</TT >; or for row-level <TT CLASS="LITERAL" >INSTEAD OF</TT > <TT CLASS="COMMAND" >INSERT</TT > or <TT CLASS="COMMAND" >UPDATE</TT > triggers where the returned row is used as the source data for <TT CLASS="COMMAND" >INSERT RETURNING</TT > or <TT CLASS="COMMAND" >UPDATE RETURNING</TT > clauses. In row-level <TT CLASS="LITERAL" >BEFORE</TT > <TT CLASS="COMMAND" >DELETE</TT > or <TT CLASS="LITERAL" >INSTEAD OF</TT > <TT CLASS="COMMAND" >DELETE</TT > triggers, returning a modified row has the same effect as returning <TT CLASS="LITERAL" >OK</TT >, that is the operation proceeds. The trigger return value is ignored for all other types of triggers. </P ><DIV CLASS="TIP" ><BLOCKQUOTE CLASS="TIP" ><P ><B >Tip: </B > The result list can be made from an array representation of the modified tuple with the <TT CLASS="LITERAL" >array get</TT > Tcl command. </P ></BLOCKQUOTE ></DIV ><P > Here's a little example trigger procedure that forces an integer value in a table to keep track of the number of updates that are performed on the row. For new rows inserted, the value is initialized to 0 and then incremented on every update operation. </P><PRE CLASS="PROGRAMLISTING" >CREATE FUNCTION trigfunc_modcount() RETURNS trigger AS $$ switch $TG_op { INSERT { set NEW($1) 0 } UPDATE { set NEW($1) $OLD($1) incr NEW($1) } default { return OK } } return [array get NEW] $$ LANGUAGE pltcl; CREATE TABLE mytab (num integer, description text, modcnt integer); CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');</PRE ><P> Notice that the trigger procedure itself does not know the column name; that's supplied from the trigger arguments. This lets the trigger procedure be reused with different tables. </P ></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="pltcl-dbaccess.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="pltcl-unknown.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Database Access from PL/Tcl</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="pltcl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Modules and the <CODE CLASS="FUNCTION" >unknown</CODE > Command</TD ></TR ></TABLE ></DIV ></BODY ></HTML >