DROP and ddl_command_end.

Hello.

When I created an event trigger for ddl_command_end, I think the only
means to identify for what the trigger function is called is
pg_event_trigger_ddl_commands() so I wrote as the following function
and defined an event trigger for ddl_command_end.

CREATE OR REPLACE FUNCTION hoge() RETURNS event_trigger AS $$
DECLARE
cmd record = pg_event_trigger_ddl_commands();
BEGIN
RAISE NOTICE '"%" "%" "%"',
cmd.command_tag, cmd.object_type, cmd.object_identity;
END
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER hoge_trigger ON ddl_command_end EXECUTE FUNCTION hoge();

Finally I got an ERROR while DROP.

=# CREATE TABLE t (a int);
NOTICE: "CREATE TABLE" "table" "public.t"
CREATE TABLE
postgres=# DROP TABLE t;
ERROR: record "cmd" is not assigned yet
DETAIL: The tuple structure of a not-yet-assigned record is indeterminate.
CONTEXT: PL/pgSQL function hoge() line 5 at RAISE

The function doesn't return a record for DROP statements.

The documentation is written as the follows:

https://postgresql.org/docs/current/event-trigger-definition.html
> 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 Section 9.28). 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.

So I think at least pg_event_trigger_ddl_command must return a record
for all commands that trigger ddl_command_end and the record should
have the correct command_tag. DROP TABLE is currently classified as
supporting event trigger. If we don't do that, any workaround and
documentation is needed.

I may be missing something, andt any opinions, thoughts or suggestions
are welcome.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center