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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>42.10. Trigger Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="plpgsql-errors-and-messages.html" title="42.9. Errors and Messages" /><link rel="next" href="plpgsql-implementation.html" title="42.11. PL/pgSQL under the Hood" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">42.10. Trigger Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpgsql-errors-and-messages.html" title="42.9. Errors and Messages">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpgsql.html" title="Chapter 42. PL/pgSQL - SQL Procedural Language">Up</a></td><th width="60%" align="center">Chapter 42. <span xmlns="http://www.w3.org/1999/xhtml" class="application">PL/pgSQL</span> - <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym> Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpgsql-implementation.html" title="42.11. PL/pgSQL under the Hood">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLPGSQL-TRIGGER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">42.10. Trigger Functions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="plpgsql-trigger.html#PLPGSQL-DML-TRIGGER">42.10.1. Triggers on Data Changes</a></span></dt><dt><span class="sect2"><a href="plpgsql-trigger.html#PLPGSQL-EVENT-TRIGGER">42.10.2. Triggers on Events</a></span></dt></dl></div><a id="id-1.8.8.12.2" class="indexterm"></a><p>
   <span class="application">PL/pgSQL</span> can be used to define trigger
   functions on data changes or database events.
   A trigger function is created with the <code class="command">CREATE FUNCTION</code>
   command, declaring it as a function with no arguments and a return type of
   <code class="type">trigger</code> (for data change triggers) or
   <code class="type">event_trigger</code> (for database event triggers).
   Special local variables named <code class="varname">TG_<em class="replaceable"><code>something</code></em></code> are
   automatically defined to describe the condition that triggered the call.
  </p><div class="sect2" id="PLPGSQL-DML-TRIGGER"><div class="titlepage"><div><div><h3 class="title">42.10.1. Triggers on Data Changes</h3></div></div></div><p>
   A <a class="link" href="triggers.html" title="Chapter 38. Triggers">data change trigger</a> is declared as a
   function with no arguments and a return type of <code class="type">trigger</code>.
   Note that the function must be declared with no arguments even if it
   expects to receive some arguments specified in <code class="command">CREATE TRIGGER</code>
   — such arguments are passed via <code class="varname">TG_ARGV</code>, 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><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="varname">NEW</code></span></dt><dd><p>
       Data type <code class="type">RECORD</code>; variable holding the new
       database row for <code class="command">INSERT</code>/<code class="command">UPDATE</code> operations in row-level
       triggers. This variable is null in statement-level triggers
       and for <code class="command">DELETE</code> operations.
      </p></dd><dt><span class="term"><code class="varname">OLD</code></span></dt><dd><p>
       Data type <code class="type">RECORD</code>; variable holding the old
       database row for <code class="command">UPDATE</code>/<code class="command">DELETE</code> operations in row-level
       triggers. This variable is null in statement-level triggers
       and for <code class="command">INSERT</code> operations.
      </p></dd><dt><span class="term"><code class="varname">TG_NAME</code></span></dt><dd><p>
       Data type <code class="type">name</code>; variable that contains the name of the trigger actually
       fired.
      </p></dd><dt><span class="term"><code class="varname">TG_WHEN</code></span></dt><dd><p>
       Data type <code class="type">text</code>; a string of
       <code class="literal">BEFORE</code>, <code class="literal">AFTER</code>, or
       <code class="literal">INSTEAD OF</code>, depending on the trigger's definition.
      </p></dd><dt><span class="term"><code class="varname">TG_LEVEL</code></span></dt><dd><p>
       Data type <code class="type">text</code>; a string of either
       <code class="literal">ROW</code> or <code class="literal">STATEMENT</code>
       depending on the trigger's definition.
      </p></dd><dt><span class="term"><code class="varname">TG_OP</code></span></dt><dd><p>
       Data type <code class="type">text</code>; a string of
       <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>,
       <code class="literal">DELETE</code>, or <code class="literal">TRUNCATE</code>
       telling for which operation the trigger was fired.
      </p></dd><dt><span class="term"><code class="varname">TG_RELID</code></span></dt><dd><p>
       Data type <code class="type">oid</code>; the object ID of the table that caused the
       trigger invocation.
      </p></dd><dt><span class="term"><code class="varname">TG_RELNAME</code></span></dt><dd><p>
       Data type <code class="type">name</code>; the name of the table that caused the trigger
       invocation. This is now deprecated, and could disappear in a future
       release. Use <code class="literal">TG_TABLE_NAME</code> instead.
      </p></dd><dt><span class="term"><code class="varname">TG_TABLE_NAME</code></span></dt><dd><p>
       Data type <code class="type">name</code>; the name of the table that
       caused the trigger invocation.
      </p></dd><dt><span class="term"><code class="varname">TG_TABLE_SCHEMA</code></span></dt><dd><p>
       Data type <code class="type">name</code>; the name of the schema of the
       table that caused the trigger invocation.
      </p></dd><dt><span class="term"><code class="varname">TG_NARGS</code></span></dt><dd><p>
       Data type <code class="type">integer</code>; the number of arguments given to the trigger
       function in the <code class="command">CREATE TRIGGER</code> statement.
      </p></dd><dt><span class="term"><code class="varname">TG_ARGV[]</code></span></dt><dd><p>
       Data type array of <code class="type">text</code>; the arguments from
       the <code class="command">CREATE TRIGGER</code> statement.
       The index counts from 0. Invalid
       indexes (less than 0 or greater than or equal to <code class="varname">tg_nargs</code>)
       result in a null value.
      </p></dd></dl></div><p>
  </p><p>
    A trigger function must return either <code class="symbol">NULL</code> or a
    record/row value having exactly the structure of the table the
    trigger was fired for.
   </p><p>
    Row-level triggers fired <code class="literal">BEFORE</code> 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
    <code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code> 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 <code class="varname">NEW</code> 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, <code class="varname">NEW</code> (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 <code class="varname">NEW</code> and return the
    modified <code class="varname">NEW</code>, or to build a complete new record/row to
    return.  In the case of a before-trigger
    on <code class="command">DELETE</code>, the returned value has no direct
    effect, but it has to be nonnull to allow the trigger action to
    proceed.  Note that <code class="varname">NEW</code> is null
    in <code class="command">DELETE</code> triggers, so returning that is
    usually not sensible.  The usual idiom in <code class="command">DELETE</code>
    triggers is to return <code class="varname">OLD</code>.
   </p><p>
    <code class="literal">INSTEAD OF</code> 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
    <code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code>).
    Otherwise a nonnull value should be returned, to signal
    that the trigger performed the requested operation. For
    <code class="command">INSERT</code> and <code class="command">UPDATE</code> operations, the return value
    should be <code class="varname">NEW</code>, which the trigger function may modify to
    support <code class="command">INSERT RETURNING</code> and <code class="command">UPDATE RETURNING</code>
    (this will also affect the row value passed to any subsequent triggers,
    or passed to a special <code class="varname">EXCLUDED</code> alias reference within
    an <code class="command">INSERT</code> statement with an <code class="literal">ON CONFLICT DO
    UPDATE</code> clause).  For <code class="command">DELETE</code> operations, the return
    value should be <code class="varname">OLD</code>.
   </p><p>
    The return value of a row-level trigger
    fired <code class="literal">AFTER</code> or a statement-level trigger
    fired <code class="literal">BEFORE</code> or <code class="literal">AFTER</code> 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 class="xref" href="plpgsql-trigger.html#PLPGSQL-TRIGGER-EXAMPLE" title="Example 42.3. A PL/pgSQL Trigger Function">Example 42.3</a> shows an example of a
    trigger function in <span class="application">PL/pgSQL</span>.
   </p><div class="example" id="PLPGSQL-TRIGGER-EXAMPLE"><p class="title"><strong>Example 42.3. A <span class="application">PL/pgSQL</span> Trigger Function</strong></p><div class="example-contents"><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 they must pay for it?
        IF NEW.salary &lt; 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 FUNCTION emp_stamp();
</pre></div></div><br class="example-break" /><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 class="xref" href="plpgsql-trigger.html#PLPGSQL-TRIGGER-AUDIT-EXAMPLE" title="Example 42.4. A PL/pgSQL Trigger Function for Auditing">Example 42.4</a> shows an example of an
    audit trigger function in <span class="application">PL/pgSQL</span>.
   </p><div class="example" id="PLPGSQL-TRIGGER-AUDIT-EXAMPLE"><p class="title"><strong>Example 42.4. A <span class="application">PL/pgSQL</span> Trigger Function for Auditing</strong></p><div class="example-contents"><p>
     This example trigger ensures that any insert, update or delete of a row
     in the <code class="literal">emp</code> table is recorded (i.e., audited) in the <code class="literal">emp_audit</code> 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,
        -- making 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.*;
        ELSIF (TG_OP = 'UPDATE') THEN
            INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
        ELSIF (TG_OP = 'INSERT') THEN
            INSERT INTO emp_audit SELECT 'I', now(), user, 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 FUNCTION process_emp_audit();
</pre></div></div><br class="example-break" /><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 class="xref" href="plpgsql-trigger.html#PLPGSQL-VIEW-TRIGGER-AUDIT-EXAMPLE" title="Example 42.5. A PL/pgSQL View Trigger Function for Auditing">Example 42.5</a> shows an example
    of an audit trigger on a view in <span class="application">PL/pgSQL</span>.
   </p><div class="example" id="PLPGSQL-VIEW-TRIGGER-AUDIT-EXAMPLE"><p class="title"><strong>Example 42.5. A <span class="application">PL/pgSQL</span> View Trigger Function for Auditing</strong></p><div class="example-contents"><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 <code class="literal">emp_audit</code> 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 FUNCTION update_emp_view();
</pre></div></div><br class="example-break" /><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 class="xref" href="plpgsql-trigger.html#PLPGSQL-TRIGGER-SUMMARY-EXAMPLE" title="Example 42.6. A PL/pgSQL Trigger Function for Maintaining a Summary Table">Example 42.6</a> shows an example of a
    trigger function in <span class="application">PL/pgSQL</span> that maintains
    a summary table for a fact table in a data warehouse.
   </p><div class="example" id="PLPGSQL-TRIGGER-SUMMARY-EXAMPLE"><p class="title"><strong>Example 42.6. A <span class="application">PL/pgSQL</span> Trigger Function for Maintaining a Summary Table</strong></p><div class="example-contents"><p>
     The schema detailed here is partly based on the <span class="emphasis"><em>Grocery Store
     </em></span> example from <span class="emphasis"><em>The Data Warehouse Toolkit</em></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 : % -&gt; % 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.
        &lt;&lt;insert_update&gt;&gt;
        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 FUNCTION 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><br class="example-break" /><p>
    <code class="literal">AFTER</code> triggers can also make use of <em class="firstterm">transition
    tables</em> to inspect the entire set of rows changed by the triggering
    statement.  The <code class="command">CREATE TRIGGER</code> command assigns names to one
    or both transition tables, and then the function can refer to those names
    as though they were read-only temporary tables.
    <a class="xref" href="plpgsql-trigger.html#PLPGSQL-TRIGGER-AUDIT-TRANSITION-EXAMPLE" title="Example 42.7. Auditing with Transition Tables">Example 42.7</a> shows an example.
   </p><div class="example" id="PLPGSQL-TRIGGER-AUDIT-TRANSITION-EXAMPLE"><p class="title"><strong>Example 42.7. Auditing with Transition Tables</strong></p><div class="example-contents"><p>
     This example produces the same results as
     <a class="xref" href="plpgsql-trigger.html#PLPGSQL-TRIGGER-AUDIT-EXAMPLE" title="Example 42.4. A PL/pgSQL Trigger Function for Auditing">Example 42.4</a>, but instead of using a
     trigger that fires for every row, it uses a trigger that fires once
     per statement, after collecting the relevant information in a transition
     table.  This can be significantly faster than the row-trigger approach
     when the invoking statement has modified many rows.  Notice that we must
     make a separate trigger declaration for each kind of event, since the
     <code class="literal">REFERENCING</code> clauses must be different for each case.  But
     this does not stop us from using a single trigger function if we choose.
     (In practice, it might be better to use three separate functions and
     avoid the run-time tests on <code class="varname">TG_OP</code>.)
    </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 rows in emp_audit to reflect the operations performed on emp,
        -- making 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, o.* FROM old_table o;
        ELSIF (TG_OP = 'UPDATE') THEN
            INSERT INTO emp_audit
                SELECT 'U', now(), user, n.* FROM new_table n;
        ELSIF (TG_OP = 'INSERT') THEN
            INSERT INTO emp_audit
                SELECT 'I', now(), user, n.* FROM new_table n;
        END IF;
        RETURN NULL; -- result is ignored since this is an AFTER trigger
    END;
$emp_audit$ LANGUAGE plpgsql;

CREATE TRIGGER emp_audit_ins
    AFTER INSERT ON emp
    REFERENCING NEW TABLE AS new_table
    FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_upd
    AFTER UPDATE ON emp
    REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
    FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_del
    AFTER DELETE ON emp
    REFERENCING OLD TABLE AS old_table
    FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
</pre></div></div><br class="example-break" /></div><div class="sect2" id="PLPGSQL-EVENT-TRIGGER"><div class="titlepage"><div><div><h3 class="title">42.10.2. Triggers on Events</h3></div></div></div><p>
    <span class="application">PL/pgSQL</span> can be used to define
    <a class="link" href="event-triggers.html" title="Chapter 39. Event Triggers">event triggers</a>.
    <span class="productname">PostgreSQL</span> requires that a function that
    is to be called as an event trigger must be declared as a function with
    no arguments and a return type of <code class="literal">event_trigger</code>.
   </p><p>
    When a <span class="application">PL/pgSQL</span> function is called as an
    event trigger, several special variables are created automatically
    in the top-level block. They are:

   </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="varname">TG_EVENT</code></span></dt><dd><p>
       Data type <code class="type">text</code>; a string representing the event the
       trigger is fired for.
      </p></dd><dt><span class="term"><code class="varname">TG_TAG</code></span></dt><dd><p>
       Data type <code class="type">text</code>; variable that contains the command tag
       for which the trigger is fired.
      </p></dd></dl></div><p>
  </p><p>
    <a class="xref" href="plpgsql-trigger.html#PLPGSQL-EVENT-TRIGGER-EXAMPLE" title="Example 42.8. A PL/pgSQL Event Trigger Function">Example 42.8</a> shows an example of an
    event trigger function in <span class="application">PL/pgSQL</span>.
   </p><div class="example" id="PLPGSQL-EVENT-TRIGGER-EXAMPLE"><p class="title"><strong>Example 42.8. A <span class="application">PL/pgSQL</span> Event Trigger Function</strong></p><div class="example-contents"><p>
     This example trigger simply raises a <code class="literal">NOTICE</code> message
     each time a supported command is executed.
    </p><pre class="programlisting">
CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
BEGIN
    RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
END;
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE FUNCTION snitch();
</pre></div></div><br class="example-break" /></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpgsql-errors-and-messages.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpgsql.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpgsql-implementation.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">42.9. Errors and Messages </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 42.11. <span class="application">PL/pgSQL</span> under the Hood</td></tr></table></div></body></html>