7.3. Event Triggers

To supplement the trigger mechanism discussed in triggers, 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.

7.3.1. 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, table_rewrite 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, DROP, SECURITY LABEL, COMMENT, GRANT or REVOKE 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. To obtain more details on the DDL operations that took place, use the set-returning function pg_event_trigger_ddl_commands() from the ddl_command_end event trigger code (see functions-event-triggers). Note that the trigger fires after the actions have taken place (but before the transaction commits), and thus the system catalogs can be read as already changed.

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 functions-event-triggers). 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 some actions of the commands ALTER TABLE and ALTER TYPE. While other control statements are available to rewrite a table, like CLUSTER and VACUUM, the table_rewrite event is not triggered by them.

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-trigger-matrix.

Event triggers are created using the command sql-createeventtrigger. 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.

7.3.2. Event Trigger Firing Matrix

event-trigger-by-command-tag lists all commands for which event triggers are supported.

Event Trigger Support by Command Tag

7.3.3. Writing Event Trigger Functions 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)-&gt;context != NULL &amp;&amp; IsA((fcinfo)-&gt;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 */
    CommandTag  tag;        /* command tag */
} EventTriggerData;

where the members are defined as follows:

Always T_EventTriggerData.
Describes the event for which the function is called, one of «ddl_command_start», «ddl_command_end», «sql_drop», «table_rewrite». See event-trigger-definition for the meaning of these events.
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.
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).

7.3.4. 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 event-trigger-definition) are prevented from running.

This is the source code of the trigger function:

#include "postgres.h"
#include "commands/event_trigger.h"


PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(noddl);

Datum
noddl(PG_FUNCTION_ARGS)
{
    EventTriggerData *trigdata;

    if (!CALLED_AS_EVENT_TRIGGER(fcinfo))  /* internal error */
        elog(ERROR, "not fired by event trigger manager");

    trigdata = (EventTriggerData *) fcinfo->context;

    ereport(ERROR,
        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("command \"%s\" denied", trigdata->tag)));

    PG_RETURN_NULL();
}

After you have compiled the source code (see dfunc), 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 FUNCTION noddl();

Now you can test the operation of the trigger:

=# \dy
                     List of event triggers
 Name  |       Event       | Owner | Enabled | Function | 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.)

7.3.5. 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 FUNCTION no_rewrite();