Horton

Re: remove spurious CREATE INDEX CONCURRENTLY wait

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2020-08-11T00:37:46Z
Lists: pgsql-hackers 
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.)

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.

BTW, what about REINDEX CONCURRENTLY?

			regards, tom lane

Commits

  1. Restore lock level to set vacuum flags

  2. Restore lock level to update statusFlags

  3. Avoid spurious waits in concurrent indexing

  4. Centralize logic for skipping useless ereport/elog calls.

  5. Don't hold ProcArrayLock longer than needed in rare cases

  6. Relax lock level for setting PGPROC->statusFlags

  7. Rename PGPROC->vacuumFlags to statusFlags

  8. snapshot scalability: Move PGXACT->vacuumFlags to ProcGlobal->vacuumFlags.

  9. Allow an autovacuum worker to be interrupted automatically when it is found