On Sun, Jul 21, 2019 at 01:34:22PM +0200, Tomas Vondra wrote: > > ... > >I wonder if we're approaching this wrong. Maybe we should not reverse >engineer queries for the various places, but just start with a set of >queries that we want to optimize, and then identify which places in the >planner need to be modified. >
I've decided to do a couple of experiments, trying to make my mind about which modified places matter to diffrent queries. But instead of trying to reverse engineer the queries, I've taken a different approach - I've compiled a list of queries that I think are sensible and relevant, and then planned them with incremental sort enabled in different places.
I don't have any clear conclusions at this point - it does show some of the places don't change plan for any of the queries, although there may be some additional query where it'd make a difference.
But I'm posting this mostly because it might be useful. I've initially planned to move changes that add incremental sort paths to separate patches, and then apply/skip different subsets of those patches. But then I realized there's a better way to do this - I've added a bunch of GUCs, one for each such place. This allows doing this testing without having to rebuild repeatedly.
I'm not going to post the patch(es) with extra GUCs here, because it'd just confuse the patch tester, but it's available here:
There are 10 GUCs, one for each place in planner where incremental sort paths are constructed. By default all those are set to 'false' so no incremental sort paths are built. If you do
SET devel_create_ordered_paths = on;
it'll start creating the paths in non-parallel in create_ordered_paths. Then you may enable devel_create_ordered_paths_parallel to also consider parallel paths, etc.
The list of queries (synthetic, but hopefully sufficiently realistic) and a couple of scripts to collect the plans is in this repository: