On Mon, Aug 10, 2020 at 8:37 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Alvaro Herrera <alvhe...@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