On 1/16/24 21:10, Konstantin Knizhnik wrote: > > ... > >> 4. I think that performing prefetch at executor level is really great >>> idea and so prefetch can be used by all indexes, including custom >>> indexes. But prefetch will be efficient only if index can provide fast >>> access to next TID (located at the same page). I am not sure that it is >>> true for all builtin indexes (GIN, GIST, BRIN,...) and especially for >>> custom AM. I wonder if we should extend AM API to make index make a >>> decision weather to perform prefetch of TIDs or not. >> I'm not against having a flag to enable/disable prefetching, but the >> question is whether doing prefetching for such indexes can be harmful. >> I'm not sure about that. > > I tend to agree with you - it is hard to imagine index implementation > which doesn't win from prefetching heap pages. > May be only the filtering case you have mentioned. But it seems to me > that current B-Tree index scan (not IOS) implementation in Postgres > doesn't try to use index tuple to check extra condition - it will fetch > heap tuple in any case. >
That's true, but that's why I started working on this: https://commitfest.postgresql.org/46/4352/ I need to think about how to combine that with the prefetching. The good thing is that both changes require fetching TIDs, not slots. I think the condition can be simply added to the prefetch callback. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
