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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</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/pgSQL - SQL Procedural Language" HREF="plpgsql.html"><LINK REL="PREVIOUS" TITLE="Errors and Messages" HREF="plpgsql-errors-and-messages.html"><LINK REL="NEXT" TITLE="PL/pgSQL Under the Hood" HREF="plpgsql-implementation.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="Errors and Messages" HREF="plpgsql-errors-and-messages.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="plpgsql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 39. <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > - <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > Procedural Language</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="PL/pgSQL Under the Hood" HREF="plpgsql-implementation.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="PLPGSQL-TRIGGER" >39.9. Trigger Procedures</A ></H1 ><P > <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > can be used to define trigger procedures. A trigger procedure is created with the <TT CLASS="COMMAND" >CREATE FUNCTION</TT > command, declaring it as a function with no arguments and a return type of <TT CLASS="TYPE" >trigger</TT >. Note that the function must be declared with no arguments even if it expects to receive arguments specified in <TT CLASS="COMMAND" >CREATE TRIGGER</TT > — trigger arguments are passed via <TT CLASS="VARNAME" >TG_ARGV</TT >, as described below. </P ><P > When a <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > function is called as a trigger, several special variables are created automatically in the top-level block. They are: <P ></P ></P><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="VARNAME" >NEW</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >RECORD</TT >; variable holding the new database row for <TT CLASS="COMMAND" >INSERT</TT >/<TT CLASS="COMMAND" >UPDATE</TT > operations in row-level triggers. This variable is <TT CLASS="SYMBOL" >NULL</TT > in statement-level triggers and for <TT CLASS="COMMAND" >DELETE</TT > operations. </P ></DD ><DT ><TT CLASS="VARNAME" >OLD</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >RECORD</TT >; variable holding the old database row for <TT CLASS="COMMAND" >UPDATE</TT >/<TT CLASS="COMMAND" >DELETE</TT > operations in row-level triggers. This variable is <TT CLASS="SYMBOL" >NULL</TT > in statement-level triggers and for <TT CLASS="COMMAND" >INSERT</TT > operations. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_NAME</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >name</TT >; variable that contains the name of the trigger actually fired. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_WHEN</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >text</TT >; a string of <TT CLASS="LITERAL" >BEFORE</TT >, <TT CLASS="LITERAL" >AFTER</TT >, or <TT CLASS="LITERAL" >INSTEAD OF</TT >, depending on the trigger's definition. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_LEVEL</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >text</TT >; a string of either <TT CLASS="LITERAL" >ROW</TT > or <TT CLASS="LITERAL" >STATEMENT</TT > depending on the trigger's definition. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_OP</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >text</TT >; a string of <TT CLASS="LITERAL" >INSERT</TT >, <TT CLASS="LITERAL" >UPDATE</TT >, <TT CLASS="LITERAL" >DELETE</TT >, or <TT CLASS="LITERAL" >TRUNCATE</TT > telling for which operation the trigger was fired. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_RELID</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >oid</TT >; the object ID of the table that caused the trigger invocation. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_RELNAME</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >name</TT >; the name of the table that caused the trigger invocation. This is now deprecated, and could disappear in a future release. Use <TT CLASS="LITERAL" >TG_TABLE_NAME</TT > instead. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_TABLE_NAME</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >name</TT >; the name of the table that caused the trigger invocation. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_TABLE_SCHEMA</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >name</TT >; the name of the schema of the table that caused the trigger invocation. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_NARGS</TT ></DT ><DD ><P > Data type <TT CLASS="TYPE" >integer</TT >; the number of arguments given to the trigger procedure in the <TT CLASS="COMMAND" >CREATE TRIGGER</TT > statement. </P ></DD ><DT ><TT CLASS="VARNAME" >TG_ARGV[]</TT ></DT ><DD ><P > Data type array of <TT CLASS="TYPE" >text</TT >; the arguments from the <TT CLASS="COMMAND" >CREATE TRIGGER</TT > statement. The index counts from 0. Invalid indexes (less than 0 or greater than or equal to <TT CLASS="VARNAME" >tg_nargs</TT >) result in a null value. </P ></DD ></DL ></DIV ><P> </P ><P > A trigger function must return either <TT CLASS="SYMBOL" >NULL</TT > or a record/row value having exactly the structure of the table the trigger was fired for. </P ><P > Row-level triggers fired <TT CLASS="LITERAL" >BEFORE</TT > can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the <TT CLASS="COMMAND" >INSERT</TT >/<TT CLASS="COMMAND" >UPDATE</TT >/<TT CLASS="COMMAND" >DELETE</TT > does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value. Returning a row value different from the original value of <TT CLASS="VARNAME" >NEW</TT > alters the row that will be inserted or updated. Thus, if the trigger function wants the triggering action to succeed normally without altering the row value, <TT CLASS="VARNAME" >NEW</TT > (or a value equal thereto) has to be returned. To alter the row to be stored, it is possible to replace single values directly in <TT CLASS="VARNAME" >NEW</TT > and return the modified <TT CLASS="VARNAME" >NEW</TT >, or to build a complete new record/row to return. In the case of a before-trigger on <TT CLASS="COMMAND" >DELETE</TT >, the returned value has no direct effect, but it has to be nonnull to allow the trigger action to proceed. Note that <TT CLASS="VARNAME" >NEW</TT > is null in <TT CLASS="COMMAND" >DELETE</TT > triggers, so returning that is usually not sensible. The usual idiom in <TT CLASS="COMMAND" >DELETE</TT > triggers is to return <TT CLASS="VARNAME" >OLD</TT >. </P ><P > <TT CLASS="LITERAL" >INSTEAD OF</TT > triggers (which are always row-level triggers, and may only be used on views) can return null to signal that they did not perform any updates, and that the rest of the operation for this row should be skipped (i.e., subsequent triggers are not fired, and the row is not counted in the rows-affected status for the surrounding <TT CLASS="COMMAND" >INSERT</TT >/<TT CLASS="COMMAND" >UPDATE</TT >/<TT CLASS="COMMAND" >DELETE</TT >). Otherwise a nonnull value should be returned, to signal that the trigger performed the requested operation. For <TT CLASS="COMMAND" >INSERT</TT > and <TT CLASS="COMMAND" >UPDATE</TT > operations, the return value should be <TT CLASS="VARNAME" >NEW</TT >, which the trigger function may modify to support <TT CLASS="COMMAND" >INSERT RETURNING</TT > and <TT CLASS="COMMAND" >UPDATE RETURNING</TT > (this will also affect the row value passed to any subsequent triggers). For <TT CLASS="COMMAND" >DELETE</TT > operations, the return value should be <TT CLASS="VARNAME" >OLD</TT >. </P ><P > The return value of a row-level trigger fired <TT CLASS="LITERAL" >AFTER</TT > or a statement-level trigger fired <TT CLASS="LITERAL" >BEFORE</TT > or <TT CLASS="LITERAL" >AFTER</TT > is always ignored; it might as well be null. However, any of these types of triggers might still abort the entire operation by raising an error. </P ><P > <A HREF="plpgsql-trigger.html#PLPGSQL-TRIGGER-EXAMPLE" >Example 39-3</A > shows an example of a trigger procedure in <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN >. </P ><DIV CLASS="EXAMPLE" ><A NAME="PLPGSQL-TRIGGER-EXAMPLE" ></A ><P ><B >Example 39-3. A <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > Trigger Procedure</B ></P ><P > This example trigger ensures that any time a row is inserted or updated in the table, the current user name and time are stamped into the row. And it checks that an employee's name is given and that the salary is a positive value. </P ><PRE CLASS="PROGRAMLISTING" >CREATE TABLE emp ( empname text, salary integer, last_date timestamp, last_user text ); CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$ BEGIN -- Check that empname and salary are given IF NEW.empname IS NULL THEN RAISE EXCEPTION 'empname cannot be null'; END IF; IF NEW.salary IS NULL THEN RAISE EXCEPTION '% cannot have null salary', NEW.empname; END IF; -- Who works for us when she must pay for it? IF NEW.salary < 0 THEN RAISE EXCEPTION '% cannot have a negative salary', NEW.empname; END IF; -- Remember who changed the payroll when NEW.last_date := current_timestamp; NEW.last_user := current_user; RETURN NEW; END; $emp_stamp$ LANGUAGE plpgsql; CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp FOR EACH ROW EXECUTE PROCEDURE emp_stamp();</PRE ></DIV ><P > Another way to log changes to a table involves creating a new table that holds a row for each insert, update, or delete that occurs. This approach can be thought of as auditing changes to a table. <A HREF="plpgsql-trigger.html#PLPGSQL-TRIGGER-AUDIT-EXAMPLE" >Example 39-4</A > shows an example of an audit trigger procedure in <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN >. </P ><DIV CLASS="EXAMPLE" ><A NAME="PLPGSQL-TRIGGER-AUDIT-EXAMPLE" ></A ><P ><B >Example 39-4. A <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > Trigger Procedure For Auditing</B ></P ><P > This example trigger ensures that any insert, update or delete of a row in the <TT CLASS="LITERAL" >emp</TT > table is recorded (i.e., audited) in the <TT CLASS="LITERAL" >emp_audit</TT > table. The current time and user name are stamped into the row, together with the type of operation performed on it. </P ><PRE CLASS="PROGRAMLISTING" >CREATE TABLE emp ( empname text NOT NULL, salary integer ); CREATE TABLE emp_audit( operation char(1) NOT NULL, stamp timestamp NOT NULL, userid text NOT NULL, empname text NOT NULL, salary integer ); CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$ BEGIN -- -- Create a row in emp_audit to reflect the operation performed on emp, -- make use of the special variable TG_OP to work out the operation. -- IF (TG_OP = 'DELETE') THEN INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*; RETURN OLD; ELSIF (TG_OP = 'UPDATE') THEN INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*; RETURN NEW; ELSIF (TG_OP = 'INSERT') THEN INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*; RETURN NEW; END IF; RETURN NULL; -- result is ignored since this is an AFTER trigger END; $emp_audit$ LANGUAGE plpgsql; CREATE TRIGGER emp_audit AFTER INSERT OR UPDATE OR DELETE ON emp FOR EACH ROW EXECUTE PROCEDURE process_emp_audit();</PRE ></DIV ><P > A variation of the previous example uses a view joining the main table to the audit table, to show when each entry was last modified. This approach still records the full audit trail of changes to the table, but also presents a simplified view of the audit trail, showing just the last modified timestamp derived from the audit trail for each entry. <A HREF="plpgsql-trigger.html#PLPGSQL-VIEW-TRIGGER-AUDIT-EXAMPLE" >Example 39-5</A > shows an example of an audit trigger on a view in <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN >. </P ><DIV CLASS="EXAMPLE" ><A NAME="PLPGSQL-VIEW-TRIGGER-AUDIT-EXAMPLE" ></A ><P ><B >Example 39-5. A <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > View Trigger Procedure For Auditing</B ></P ><P > This example uses a trigger on the view to make it updatable, and ensure that any insert, update or delete of a row in the view is recorded (i.e., audited) in the <TT CLASS="LITERAL" >emp_audit</TT > table. The current time and user name are recorded, together with the type of operation performed, and the view displays the last modified time of each row. </P ><PRE CLASS="PROGRAMLISTING" >CREATE TABLE emp ( empname text PRIMARY KEY, salary integer ); CREATE TABLE emp_audit( operation char(1) NOT NULL, userid text NOT NULL, empname text NOT NULL, salary integer, stamp timestamp NOT NULL ); CREATE VIEW emp_view AS SELECT e.empname, e.salary, max(ea.stamp) AS last_updated FROM emp e LEFT JOIN emp_audit ea ON ea.empname = e.empname GROUP BY 1, 2; CREATE OR REPLACE FUNCTION update_emp_view() RETURNS TRIGGER AS $$ BEGIN -- -- Perform the required operation on emp, and create a row in emp_audit -- to reflect the change made to emp. -- IF (TG_OP = 'DELETE') THEN DELETE FROM emp WHERE empname = OLD.empname; IF NOT FOUND THEN RETURN NULL; END IF; OLD.last_updated = now(); INSERT INTO emp_audit VALUES('D', user, OLD.*); RETURN OLD; ELSIF (TG_OP = 'UPDATE') THEN UPDATE emp SET salary = NEW.salary WHERE empname = OLD.empname; IF NOT FOUND THEN RETURN NULL; END IF; NEW.last_updated = now(); INSERT INTO emp_audit VALUES('U', user, NEW.*); RETURN NEW; ELSIF (TG_OP = 'INSERT') THEN INSERT INTO emp VALUES(NEW.empname, NEW.salary); NEW.last_updated = now(); INSERT INTO emp_audit VALUES('I', user, NEW.*); RETURN NEW; END IF; END; $$ LANGUAGE plpgsql; CREATE TRIGGER emp_audit INSTEAD OF INSERT OR UPDATE OR DELETE ON emp_view FOR EACH ROW EXECUTE PROCEDURE update_emp_view();</PRE ></DIV ><P > One use of triggers is to maintain a summary table of another table. The resulting summary can be used in place of the original table for certain queries — often with vastly reduced run times. This technique is commonly used in Data Warehousing, where the tables of measured or observed data (called fact tables) might be extremely large. <A HREF="plpgsql-trigger.html#PLPGSQL-TRIGGER-SUMMARY-EXAMPLE" >Example 39-6</A > shows an example of a trigger procedure in <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > that maintains a summary table for a fact table in a data warehouse. </P ><DIV CLASS="EXAMPLE" ><A NAME="PLPGSQL-TRIGGER-SUMMARY-EXAMPLE" ></A ><P ><B >Example 39-6. A <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > Trigger Procedure For Maintaining A Summary Table</B ></P ><P > The schema detailed here is partly based on the <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >Grocery Store </I ></SPAN > example from <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >The Data Warehouse Toolkit</I ></SPAN > by Ralph Kimball. </P ><PRE CLASS="PROGRAMLISTING" >-- -- Main tables - time dimension and sales fact. -- CREATE TABLE time_dimension ( time_key integer NOT NULL, day_of_week integer NOT NULL, day_of_month integer NOT NULL, month integer NOT NULL, quarter integer NOT NULL, year integer NOT NULL ); CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key); CREATE TABLE sales_fact ( time_key integer NOT NULL, product_key integer NOT NULL, store_key integer NOT NULL, amount_sold numeric(12,2) NOT NULL, units_sold integer NOT NULL, amount_cost numeric(12,2) NOT NULL ); CREATE INDEX sales_fact_time ON sales_fact(time_key); -- -- Summary table - sales by time. -- CREATE TABLE sales_summary_bytime ( time_key integer NOT NULL, amount_sold numeric(15,2) NOT NULL, units_sold numeric(12) NOT NULL, amount_cost numeric(15,2) NOT NULL ); CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key); -- -- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE. -- CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER AS $maint_sales_summary_bytime$ DECLARE delta_time_key integer; delta_amount_sold numeric(15,2); delta_units_sold numeric(12); delta_amount_cost numeric(15,2); BEGIN -- Work out the increment/decrement amount(s). IF (TG_OP = 'DELETE') THEN delta_time_key = OLD.time_key; delta_amount_sold = -1 * OLD.amount_sold; delta_units_sold = -1 * OLD.units_sold; delta_amount_cost = -1 * OLD.amount_cost; ELSIF (TG_OP = 'UPDATE') THEN -- forbid updates that change the time_key - -- (probably not too onerous, as DELETE + INSERT is how most -- changes will be made). IF ( OLD.time_key != NEW.time_key) THEN RAISE EXCEPTION 'Update of time_key : % -> % not allowed', OLD.time_key, NEW.time_key; END IF; delta_time_key = OLD.time_key; delta_amount_sold = NEW.amount_sold - OLD.amount_sold; delta_units_sold = NEW.units_sold - OLD.units_sold; delta_amount_cost = NEW.amount_cost - OLD.amount_cost; ELSIF (TG_OP = 'INSERT') THEN delta_time_key = NEW.time_key; delta_amount_sold = NEW.amount_sold; delta_units_sold = NEW.units_sold; delta_amount_cost = NEW.amount_cost; END IF; -- Insert or update the summary row with the new values. <<insert_update>> LOOP UPDATE sales_summary_bytime SET amount_sold = amount_sold + delta_amount_sold, units_sold = units_sold + delta_units_sold, amount_cost = amount_cost + delta_amount_cost WHERE time_key = delta_time_key; EXIT insert_update WHEN found; BEGIN INSERT INTO sales_summary_bytime ( time_key, amount_sold, units_sold, amount_cost) VALUES ( delta_time_key, delta_amount_sold, delta_units_sold, delta_amount_cost ); EXIT insert_update; EXCEPTION WHEN UNIQUE_VIOLATION THEN -- do nothing END; END LOOP insert_update; RETURN NULL; END; $maint_sales_summary_bytime$ LANGUAGE plpgsql; CREATE TRIGGER maint_sales_summary_bytime AFTER INSERT OR UPDATE OR DELETE ON sales_fact FOR EACH ROW EXECUTE PROCEDURE maint_sales_summary_bytime(); INSERT INTO sales_fact VALUES(1,1,1,10,3,15); INSERT INTO sales_fact VALUES(1,2,1,20,5,35); INSERT INTO sales_fact VALUES(2,2,1,40,15,135); INSERT INTO sales_fact VALUES(2,3,1,10,1,13); SELECT * FROM sales_summary_bytime; DELETE FROM sales_fact WHERE product_key = 1; SELECT * FROM sales_summary_bytime; UPDATE sales_fact SET units_sold = units_sold * 2; SELECT * FROM sales_summary_bytime;</PRE ></DIV ></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="plpgsql-errors-and-messages.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="plpgsql-implementation.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Errors and Messages</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="plpgsql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > Under the Hood</TD ></TR ></TABLE ></DIV ></BODY ></HTML >