Event Triggers
event trigger
To supplement the trigger mechanism discussed in ,
PostgreSQL also provides event triggers. Unlike regular
triggers, which are attached to a single table and capture only DML events,
event triggers are global to a particular database and are capable of
capturing DDL events.
Like regular triggers, event triggers can be written in any procedural
language that includes event trigger support, or in C, but not in plain
SQL.
Overview of Event Trigger Behavior
An event trigger fires whenever the event with which it is associated
occurs in the database in which it is defined. Currently, the only
supported events are
ddl_command_start,
ddl_command_end
and sql_drop.
Support for additional events may be added in future releases.
The ddl_command_start event occurs just before the
execution of a CREATE, ALTER, or DROP
command. No check whether the affected object exists or doesn't exist is
performed before the event trigger fires.
As an exception, however, this event does not occur for
DDL commands targeting shared objects — databases, roles, and tablespaces
— or for commands targeting event triggers themselves. The event trigger
mechanism does not support these object types.
ddl_command_start also occurs just before the execution of a
SELECT INTO command, since this is equivalent to
CREATE TABLE AS.
The ddl_command_end event occurs just after the execution of
this same set of commands.
The sql_drop event occurs just before the
ddl_command_end event trigger for any operation that drops
database objects. To list the objects that have been dropped, use the
set-returning function pg_event_trigger_dropped_objects() from the
sql_drop event trigger code (see
). Note that
the trigger is executed after the objects have been deleted from the
system catalogs, so it's not possible to look them up anymore.
The table_rewrite event occurs just before a table is
rewritten by the command ALTER TABLE. While other
control statements are available to rewrite a table,
like CLUSTER and VACUUM,
the table_rewrite event is currently only triggered by
the ALTER TABLE command, and only when that command
attempts to rewrite the table.
Event triggers (like other functions) cannot be executed in an aborted
transaction. Thus, if a DDL command fails with an error, any associated
ddl_command_end triggers will not be executed. Conversely,
if a ddl_command_start trigger fails with an error, no
further event triggers will fire, and no attempt will be made to execute
the command itself. Similarly, if a ddl_command_end trigger
fails with an error, the effects of the DDL statement will be rolled
back, just as they would be in any other case where the containing
transaction aborts.
For a complete list of commands supported by the event trigger mechanism,
see .
Event triggers are created using the command .
In order to create an event trigger, you must first create a function with
the special return type event_trigger. This function
need not (and may not) return a value; the return type serves merely as
a signal that the function is to be invoked as an event trigger.
If more than one event trigger is defined for a particular event, they will
fire in alphabetical order by trigger name.
A trigger definition can also specify a WHEN
condition so that, for example, a ddl_command_start
trigger can be fired only for particular commands which the user wishes
to intercept. A common use of such triggers is to restrict the range of
DDL operations which users may perform.
Event Trigger Firing Matrix
lists all commands
for which event triggers are supported.
Event Trigger Support by Command Tag
command tag
ddl_command_start
ddl_command_end
sql_drop
table_rewrite
ALTER AGGREGATE
X
X
-
-
ALTER COLLATION
X
X
-
-
ALTER CONVERSION
X
X
-
-
ALTER DOMAIN
X
X
-
-
ALTER EXTENSION
X
X
-
-
ALTER FOREIGN DATA WRAPPER
X
X
-
-
ALTER FOREIGN TABLE
X
X
X
-
ALTER FUNCTION
X
X
-
-
ALTER LANGUAGE
X
X
-
-
ALTER OPERATOR
X
X
-
-
ALTER OPERATOR CLASS
X
X
-
-
ALTER OPERATOR FAMILY
X
X
-
-
ALTER POLICY
X
X
-
-
ALTER SCHEMA
X
X
-
-
ALTER SEQUENCE
X
X
-
-
ALTER SERVER
X
X
-
-
ALTER TABLE
X
X
X
X
ALTER TEXT SEARCH CONFIGURATION
X
X
-
-
ALTER TEXT SEARCH DICTIONARY
X
X
-
-
ALTER TEXT SEARCH PARSER
X
X
-
-
ALTER TEXT SEARCH TEMPLATE
X
X
-
-
ALTER TRIGGER
X
X
-
-
ALTER TYPE
X
X
-
-
ALTER USER MAPPING
X
X
-
-
ALTER VIEW
X
X
-
-
CREATE AGGREGATE
X
X
-
-
CREATE CAST
X
X
-
-
CREATE COLLATION
X
X
-
-
CREATE CONVERSION
X
X
-
-
CREATE DOMAIN
X
X
-
-
CREATE EXTENSION
X
X
-
-
CREATE FOREIGN DATA WRAPPER
X
X
-
-
CREATE FOREIGN TABLE
X
X
-
-
CREATE FUNCTION
X
X
-
-
CREATE INDEX
X
X
-
-
CREATE LANGUAGE
X
X
-
-
CREATE OPERATOR
X
X
-
-
CREATE OPERATOR CLASS
X
X
-
-
CREATE OPERATOR FAMILY
X
X
-
-
CREATE POLICY
X
X
-
-
CREATE RULE
X
X
-
-
CREATE SCHEMA
X
X
-
-
CREATE SEQUENCE
X
X
-
-
CREATE SERVER
X
X
-
-
CREATE TABLE
X
X
-
-
CREATE TABLE AS
X
X
-
-
CREATE TEXT SEARCH CONFIGURATION
X
X
-
-
CREATE TEXT SEARCH DICTIONARY
X
X
-
-
CREATE TEXT SEARCH PARSER
X
X
-
-
CREATE TEXT SEARCH TEMPLATE
X
X
-
-
CREATE TRIGGER
X
X
-
-
CREATE TYPE
X
X
-
-
CREATE USER MAPPING
X
X
-
-
CREATE VIEW
X
X
-
-
DROP AGGREGATE
X
X
X
-
DROP CAST
X
X
X
-
DROP COLLATION
X
X
X
-
DROP CONVERSION
X
X
X
-
DROP DOMAIN
X
X
X
-
DROP EXTENSION
X
X
X
-
DROP FOREIGN DATA WRAPPER
X
X
X
-
DROP FOREIGN TABLE
X
X
X
-
DROP FUNCTION
X
X
X
-
DROP INDEX
X
X
X
-
DROP LANGUAGE
X
X
X
-
DROP OPERATOR
X
X
X
-
DROP OPERATOR CLASS
X
X
X
-
DROP OPERATOR FAMILY
X
X
X
-
DROP OWNED
X
X
X
-
DROP POLICY
X
X
X
-
DROP RULE
X
X
X
-
DROP SCHEMA
X
X
X
-
DROP SEQUENCE
X
X
X
-
DROP SERVER
X
X
X
-
DROP TABLE
X
X
X
-
DROP TEXT SEARCH CONFIGURATION
X
X
X
-
DROP TEXT SEARCH DICTIONARY
X
X
X
-
DROP TEXT SEARCH PARSER
X
X
X
-
DROP TEXT SEARCH TEMPLATE
X
X
X
-
DROP TRIGGER
X
X
X
-
DROP TYPE
X
X
X
-
DROP USER MAPPING
X
X
X
-
DROP VIEW
X
X
X
-
IMPORT FOREIGN SCHEMA
X
X
-
-
SELECT INTO
X
X
-
-
Writing Event Trigger Functions in C
event trigger
in C
This section describes the low-level details of the interface to an
event trigger function. This information is only needed when writing
event trigger functions in C. If you are using a higher-level language
then these details are handled for you. In most cases you should
consider using a procedural language before writing your event triggers
in C. The documentation of each procedural language explains how to
write an event trigger in that language.
Event trigger functions must use the version 1 function
manager interface.
When a function is called by the event trigger manager, it is not passed
any normal arguments, but it is passed a context pointer
pointing to a EventTriggerData structure. C functions can
check whether they were called from the event trigger manager or not by
executing the macro:
CALLED_AS_EVENT_TRIGGER(fcinfo)
which expands to:
((fcinfo)->context != NULL && IsA((fcinfo)->context, EventTriggerData))
If this returns true, then it is safe to cast
fcinfo->context to type EventTriggerData
* and make use of the pointed-to
EventTriggerData structure. The function must
not alter the EventTriggerData
structure or any of the data it points to.
struct EventTriggerData is defined in
commands/event_trigger.h:
typedef struct EventTriggerData
{
NodeTag type;
const char *event; /* event name */
Node *parsetree; /* parse tree */
const char *tag; /* command tag */
} EventTriggerData;
where the members are defined as follows:
type
Always T_EventTriggerData.
event
Describes the event for which the function is called, one of
"ddl_command_start", "ddl_command_end",
"sql_drop".
See for the meaning of these
events.
parsetree
A pointer to the parse tree of the command. Check the PostgreSQL
source code for details. The parse tree structure is subject to change
without notice.
tag
The command tag associated with the event for which the event trigger
is run, for example "CREATE FUNCTION".
An event trigger function must return a NULL pointer
(not an SQL null value, that is, do not
set isNull true).
A Complete Event Trigger Example
Here is a very simple example of an event trigger function written in C.
(Examples of triggers written in procedural languages can be found in
the documentation of the procedural languages.)
The function noddl raises an exception each time it is called.
The event trigger definition associated the function with
the ddl_command_start event. The effect is that all DDL
commands (with the exceptions mentioned
in ) are prevented from running.
This is the source code of the trigger function:
context;
ereport(ERROR,
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
errmsg("command \"%s\" denied", trigdata->tag)));
PG_RETURN_NULL();
}
]]>
After you have compiled the source code (see ),
declare the function and the triggers:
CREATE FUNCTION noddl() RETURNS event_trigger
AS 'noddl' LANGUAGE C;
CREATE EVENT TRIGGER noddl ON ddl_command_start
EXECUTE PROCEDURE noddl();
Now you can test the operation of the trigger:
=# \dy
List of event triggers
Name | Event | Owner | Enabled | Procedure | Tags
-------+-------------------+-------+---------+-----------+------
noddl | ddl_command_start | dim | enabled | noddl |
(1 row)
=# CREATE TABLE foo(id serial);
ERROR: command "CREATE TABLE" denied
In this situation, in order to be able to run some DDL commands when you
need to do so, you have to either drop the event trigger or disable it. It
can be convenient to disable the trigger for only the duration of a
transaction:
BEGIN;
ALTER EVENT TRIGGER noddl DISABLE;
CREATE TABLE foo (id serial);
ALTER EVENT TRIGGER noddl ENABLE;
COMMIT;
(Recall that DDL commands on event triggers themselves are not affected by
event triggers.)
A Table Rewrite Event Trigger Example
Thanks to the table_rewrite event, it is possible to implement
a table rewriting policy only allowing the rewrite in maintenance windows.
Here's an example implementing such a policy.
CREATE OR REPLACE FUNCTION no_rewrite()
RETURNS event_trigger
LANGUAGE plpgsql AS
$$
---
--- Implement local Table Rewriting policy:
--- public.foo is not allowed rewriting, ever
--- other tables are only allowed rewriting between 1am and 6am
--- unless they have more than 100 blocks
---
DECLARE
table_oid oid := pg_event_trigger_table_rewrite_oid();
current_hour integer := extract('hour' from current_time);
pages integer;
max_pages integer := 100;
BEGIN
IF pg_event_trigger_table_rewrite_oid() = 'public.foo'::regclass
THEN
RAISE EXCEPTION 'you''re not allowed to rewrite the table %',
table_oid::regclass;
END IF;
SELECT INTO pages relpages FROM pg_class WHERE oid = table_oid;
IF pages > max_pages
THEN
RAISE EXCEPTION 'rewrites only allowed for table with less than % pages',
max_pages;
END IF;
IF current_hour NOT BETWEEN 1 AND 6
THEN
RAISE EXCEPTION 'rewrites only allowed between 1am and 6am';
END IF;
END;
$$;
CREATE EVENT TRIGGER no_rewrite_allowed
ON table_rewrite
EXECUTE PROCEDURE no_rewrite();