Re: Support TRUNCATE triggers on foreign tables

2022年7月8日(金) 14:06 Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>:
> On 2022/07/08 11:19, Yugo NAGATA wrote:
> >> You added "foreign tables" for BEFORE statement-level trigger as the above, but ISTM that you also needs to do that for AFTER statement-level trigger. No?
> >
> > Oops, I forgot it. I attached the updated patch.
>
> Thanks for updating the patch! LGTM.
> Barring any objection, I will commit the patch.

An observation: as-is the patch would make it possible to create a truncate
trigger for a foreign table whose FDW doesn't support truncation, which seems
somewhat pointless, possible source of confusion etc.:

postgres=# CREATE TRIGGER ft_trigger
AFTER TRUNCATE ON fb_foo
EXECUTE FUNCTION fb_foo_trg();
CREATE TRIGGER

postgres=# TRUNCATE fb_foo;
ERROR: cannot truncate foreign table "fb_foo"

It would be easy enough to check for this, e.g.:

else if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
{
FdwRoutine *fdwroutine = GetFdwRoutineForRelation(rel, false);

if (!fdwroutine->ExecForeignTruncate)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("foreign data wrapper does not support
table truncation")));
...

which results in:

postgres=# CREATE TRIGGER ft_trigger
AFTER TRUNCATE ON fb_foo
EXECUTE FUNCTION fb_foo_trg();
ERROR: foreign data wrapper does not support table truncation

which IMO is preferable to silently accepting DDL which will never
actually do anything.

Regards

Ian Barwick