Another thing to consider is the visibility map. From what I understand, index only scans are preferred for heavily updated tables, not infrequently updated ones. Even though index only scans imply ONLY they really aren't in the sense that they may need to visit the Visibility Map for the heap. This can be costly and the planner may remove index only scan consideration if the VM has tuples that are not visible.
BTW, to Andrew, the UNION ALL alternative still results in bitmap index scans from my testing.
Regards,
Michael Vitale
Jeff Janes wrote on 10/12/2019 11:17 AM:On Sat, Oct 12, 2019 at 04:39:56PM +0200, Ivan Voras wrote:
> With seqscan disabled, I get this plan on 9.6:
> Bitmap Heap Scan on friend (cost=8.42..19.01 rows=14 width=8)
...
> I expected to get an index-only scan in this situation, as that would be a
> very common query. Is there a way to actually make this sort of query
> resolvable with an index-only scan? Maybe a different table structure would
> help?
It would have to scan the entire index to find the cases where user2_id=42 but user1_id is not constrained. Technically User1_id would be constrained to be less than 42, but I don't think the planner will take that into account.
The v11 release notes have this relevant item:
https://www.postgresql.org/docs/11/release-11.html
|Allow bitmap scans to perform index-only scans when possible (Alexander Kuzmenkov)
But this is not one of those cases. It is only possible when the only data needed is whether the row exists or not.
Cheers,
Jeff