40.9. Trigger Procedures
40.9.1. Triggers on data changes
PL/pgSQL can be used to define trigger procedures. A trigger procedure is created with the CREATE FUNCTION command, declaring it as a function with no arguments and a return type of trigger. Note that the function must be declared with no arguments even if it expects to receive arguments specified in CREATE TRIGGER — trigger arguments are passed via TG_ARGV, as described below.
When a PL/pgSQL function is called as a trigger, several special variables are created automatically in the top-level block. They are:
- NEW
-
Data type RECORD; variable holding the new database row for INSERT/UPDATE operations in row-level triggers. This variable is unassigned in statement-level triggers and for DELETE operations.
- OLD
-
Data type RECORD; variable holding the old database row for UPDATE/DELETE operations in row-level triggers. This variable is unassigned in statement-level triggers and for INSERT operations.
- TG_NAME
-
Data type name; variable that contains the name of the trigger actually fired.
- TG_WHEN
-
Data type text; a string of BEFORE, AFTER, or INSTEAD OF, depending on the trigger's definition.
- TG_LEVEL
-
Data type text; a string of either ROW or STATEMENT depending on the trigger's definition.
- TG_OP
-
Data type text; a string of INSERT, UPDATE, DELETE, or TRUNCATE telling for which operation the trigger was fired.
- TG_RELID
-
Data type oid; the object ID of the table that caused the trigger invocation.
- TG_RELNAME
-
Data type name; the name of the table that caused the trigger invocation. This is now deprecated, and could disappear in a future release. Use TG_TABLE_NAME instead.
- TG_TABLE_NAME
-
Data type name; the name of the table that caused the trigger invocation.
- TG_TABLE_SCHEMA
-
Data type name; the name of the schema of the table that caused the trigger invocation.
- TG_NARGS
-
Data type integer; the number of arguments given to the trigger procedure in the CREATE TRIGGER statement.
- TG_ARGV[]
-
Data type array of text; the arguments from the CREATE TRIGGER statement. The index counts from 0. Invalid indexes (less than 0 or greater than or equal to tg_nargs) result in a null value.
A trigger function must return either NULL or a record/row value having exactly the structure of the table the trigger was fired for.
Row-level triggers fired BEFORE 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 INSERT/UPDATE/DELETE 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 NEW 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, NEW (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 NEW and return the modified NEW, or to build a complete new record/row to return. In the case of a before-trigger on DELETE, the returned value has no direct effect, but it has to be nonnull to allow the trigger action to proceed. Note that NEW is null in DELETE triggers, so returning that is usually not sensible. The usual idiom in DELETE triggers is to return OLD.
INSTEAD OF 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 INSERT/UPDATE/DELETE). Otherwise a nonnull value should be returned, to signal that the trigger performed the requested operation. For INSERT and UPDATE operations, the return value should be NEW, which the trigger function may modify to support INSERT RETURNING and UPDATE RETURNING (this will also affect the row value passed to any subsequent triggers). For DELETE operations, the return value should be OLD.
The return value of a row-level trigger fired AFTER or a statement-level trigger fired BEFORE or AFTER 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.
Example 40-3 shows an example of a trigger procedure in PL/pgSQL.
Example 40-3. A PL/pgSQL Trigger Procedure
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.
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();
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. Example 40-4 shows an example of an audit trigger procedure in PL/pgSQL.
Example 40-4. A PL/pgSQL Trigger Procedure For Auditing
This example trigger ensures that any insert, update or delete of a row in the emp table is recorded (i.e., audited) in the emp_audit table. The current time and user name are stamped into the row, together with the type of operation performed on it.
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();
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. Example 40-5 shows an example of an audit trigger on a view in PL/pgSQL.
Example 40-5. A PL/pgSQL View Trigger Procedure For Auditing
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 emp_audit 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.
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();
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. Example 40-6 shows an example of a trigger procedure in PL/pgSQL that maintains a summary table for a fact table in a data warehouse.
Example 40-6. A PL/pgSQL Trigger Procedure For Maintaining A Summary Table
The schema detailed here is partly based on the Grocery Store example from The Data Warehouse Toolkit by Ralph Kimball.
--
-- 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;
40.9.2. Triggers on events
PL/pgSQL can be used to define event triggers. PostgreSQL requires that a procedure that is to be called as an event trigger must be declared as a function with no arguments and a return type of event_trigger.
When a PL/pgSQL function is called as a event trigger, several special variables are created automatically in the top-level block. They are:
- TG_EVENT
-
Data type text; a string representing the event the trigger is fired for.
- TG_TAG
-
Data type text; variable that contains the command tag for which the trigger is fired.
Example 40-7 shows an example of a event trigger procedure in PL/pgSQL.
Example 40-7. A PL/pgSQL Event Trigger Procedure
This example trigger simply raises a NOTICE message each time a supported command is executed.
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 PROCEDURE snitch();