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>Chapter 39. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
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> Procedural Language</TD
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><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
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