Re: Faster "SET search_path"

On Mon, Aug 7, 2023 at 7:24 PM Jeff Davis <pgsql@j-davis.com> wrote:
> I might just avoid guc.c entirely and directly set
> namespace_search_path and baseSearchPathValid=false. The main thing we
> lose there is the GUC stack (AtEOXact_GUC()), but there's already a
> PG_TRY/PG_FINALLY block in fmgr_security_definer, so we can use that to
> change it back safely. (I think? I need to convince myself that it's
> safe to skip the work in guc.c, at least for the special case of
> search_path, and that it won't be too fragile.)

I suspect that dodging the GUC stack machinery is not a very good
idea. The timing of when TRY/CATCH blocks are called is different from
when subtransactions are aborted, and that distinction has messed me
up more than once when trying to code various things. Specifically,
changes that happen in a CATCH block happen before we actually reach
Abort(Sub)Transaction, which sort of makes it sound like you'd be
reverting back to the old value too early. The GUC stack is also
pretty complicated because of the SET/SAVE and LOCAL/non-LOCAL stuff.
I can't say there isn't a way to make something like what you're
talking about here work, but I bet it will be difficult to get all of
the corner cases right, and I suspect that trying to speed up the
existing mechanism is a better plan than trying to go around it.

-- 
Robert Haas
EDB: http://www.enterprisedb.com