On Wed, Sep 04, 2019 at 11:37:48AM +0200, Rafia Sabih wrote:
>On Tue, 30 Jul 2019 at 02:17, Tomas Vondra <tomas.vondra@2ndquadrant.com>
>wrote:
>
>> 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:
>>
>> https://github.com/tvondra/postgres/tree/incremental-sort-20190730
>>
>> 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:
>>
>> https://github.com/tvondra/incremental-sort-tests-2
>>
>> There's also a spreadsheet with a summary of results, with a visual
>> representation of which GUCs affect which queries.
>>
> Wow, that sounds like an elaborate experiment. But where is this
> spreadsheet you mentioned ?
>
It seems I forgot to push the commit containing the spreadsheet with
results. I'll fix that tomorrow.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services