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 */
CommandTag 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", "table_rewrite".
See Section 42.1 for the meaning of these
events.
parsetreeA 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).