Re: Async commands (like drop index)

Neil Conway wrote:
> On Fri, 2007-18-05 at 11:47 -0500, Jim C. Nasby wrote:
> > Assuming the concurrent psql stuff gets in, do you still see a use for
> > this?
>
> I think concurrent psql (and/or async libpq) is the right way to handle
> this sort of requirement. "DROP INDEX NOWAIT" is hacky, and would be
> difficult (impossible?) to implement in a reasonable manner: the backend
> is fundamentally single-threaded. Also, how does the client learn when
> the DROP INDEX actually finishes? The client would either need to poll
> the database, or we'd need to implement something like select() --
> neither is a very appealing alternative.

I think what Joshua really wants is an equivalent of this:

start:
BEGIN;
LOCK TABLE foo IN ACCESS EXCLUSIVE MODE NOWAIT;
-- if fail, rollback and go to start
DROP INDEX foo_idx;
COMMIT;

The idea is that the lock is only acquired if immediately available,
thus not blocking other queries which would otherwise be blocked behind
the DROP INDEX.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support