Re: remove spurious CREATE INDEX CONCURRENTLY wait

On Mon, Aug 10, 2020 at 8:37 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > To recap: currently, any CREATE INDEX CONCURRENTLY will wait for all
> > other CICs running concurrently to finish, because they can't be
> > distinguished amidst other old snapshots.  We can change things by
> > having CIC set a special flag in PGPROC (like PROC_IN_VACUUM) indicating
> > that it's doing CIC; other CICs will see that flag and will know that
> > they don't need to wait for those processes.  With this, CIC on small
> > tables don't have to wait for CIC on large tables to complete.
>
> Hm.  +1 for improving this, if we can, but ...
>
> It seems clearly unsafe to ignore a CIC that is in active index-building;
> a snapshot held for that purpose is just as real as any other.  It *might*
> be all right to ignore a CIC that is just waiting, but you haven't made
> any argument in the patch comments as to why that's safe either.
> (Moreover, at the points where we're just waiting, I don't think we have
> a snapshot, so another CIC's WaitForOlderSnapshots shouldn't wait for us
> anyway.)

Why is a CIC in active index-building something we need to wait for?
Wouldn't it fall under a similar kind of logic to the other snapshot
types we can explicitly ignore? CIC can't be run in a manual
transaction, so the snapshot it holds won't be used to perform
arbitrary operations (i.e., the reason why a manual ANALYZE can't be
ignored).

> Actually, it doesn't look like you've touched the comments at all.
> WaitForOlderSnapshots' header comment has a long explanation of why
> it's safe to ignore certain processes.  That certainly needs to be
> updated by any patch that's going to change the rules.

Agreed that the comment needs to be updated to discuss the
(im)possibility of arbitrary operations within a snapshot held by CIC.

James