Re: remove spurious CREATE INDEX CONCURRENTLY wait

Michael Paquier <michael@paquier.xyz> writes:
> On Mon, Nov 09, 2020 at 08:32:13PM -0500, Tom Lane wrote:
>> Do we really need exclusive lock on the ProcArray to make this flag
>> change?  That seems pretty bad from a concurrency standpoint.

> Any place where we update vacuumFlags acquires an exclusive LWLock on
> ProcArrayLock.  That's held for a very short time, so IMO it won't
> matter much in practice, particularly if you compare that with the
> potential gains related to the existing wait phases.

Not sure I believe that it doesn't matter much in practice.  If there's
a steady stream of shared ProcArrayLock acquisitions (for snapshot
acquisition) then somebody wanting exclusive lock will create a big
hiccup, whether they hold it for a short time or not.

            regards, tom lane