Re: Support for REINDEX CONCURRENTLY

Michael Paquier <michael.paquier@gmail.com> writes:
> OK, so the problem here is that the relcache, as the syscache, are relying
> on SnapshotNow which cannot be used safely as the false index definition
> could be read by other backends.

That's one problem.  It's definitely not the only one, if we're trying
to change an index's definition while an index-accessing operation is in
progress.

> I assume that the switch phase is not the longest phase of the concurrent
> operation, as you also need to build and validate the new index at prior
> steps. I am just wondering if it is acceptable to you guys to take a
> stronger lock only during this switch phase.

We might be forced to fall back on such a solution, but it's pretty
undesirable.  Even though the exclusive lock would only need to be held
for a short time, it can create a big hiccup in processing.  The key
reason is that once the ex-lock request is queued, it blocks ordinary
operations coming in behind it.  So effectively it's stopping operations
not just for the length of time the lock is *held*, but for the length
of time it's *awaited*, which could be quite long.

Note that allowing subsequent requests to jump the queue would not be a
good fix for this; if you do that, it's likely the ex-lock will never be
granted, at least not till the next system idle time.  Which if you've
got one, you don't need a feature like this at all; you might as well
just reindex normally during your idle time.
        regards, tom lane