> Here is finally a new version of the patch, where all the mentioned issues
> seems to be fixed and the corresponding new tests should keep it like that
> (I've skipped all the pubs at PostgresLondon for that).
Thanks for the new patch! Really appreciate the work you're putting into it.
I verified that the backwards index scan is indeed functioning now. However, I'm afraid it's not that simple, as I think the cursor case is broken now. I think having just the 'scan direction' in the btree code is not enough to get this working properly, because we need to know whether we want the minimum or maximum element of a certain prefix. There are basically four cases:
- Forward Index Scan + Forward cursor: we want the minimum element within a prefix and we want to skip 'forward' to the next prefix
- Forward Index Scan + Backward cursor: we want the minimum element within a prefix and we want to skip 'backward' to the previous prefix
- Backward Index Scan + Forward cursor: we want the maximum element within a prefix and we want to skip 'backward' to the previous prefix
- Backward Index Scan + Backward cursor: we want the maximum element within a prefix and we want to skip 'forward' to the next prefix
These cases make it rather complicated unfortunately. They do somewhat tie in with the previous discussion on this thread about being able to skip to the min or max value. If we ever want to support a sort of minmax scan, we'll encounter the same issues.
Also, I think in planner.c, line 4831, we should actually be looking at whether uniq_distinct_pathkeys is NIL, rather than the current check for distinct_pathkeys. That'll make the planner pick the skip scan even with queries like "select distinct on (a) a,b where a=2". Currently, it doesn't pick the skip scan here, beacuse distinct_pathkeys does not contain "a" anymore. This leads to it scanning every item for a=2 even though it can stop after the first one.
I'll do some more tests with the patch.
-Floris