On Tue, Sep 01, 2020 at 01:15:31PM +0200, Dmitry Dolgov wrote:
Hi,
Better late than never, to follow up on the original thread [1] I would like to
continue the discussion with the another version of the patch for group by
reordering optimization. To remind, it's about reordering of group by clauses
to do sorting more efficiently. The patch is rebased and modified to address
(at least partially) the suggestions about making it consider new additional
paths instead of changing original ones. It is still pretty much
proof-of-concept version though with many blind spots, but I wanted to start
kicking it and post at least something, otherwise it will never happen. An
incremental approach so to say.
In many ways it still contains the original code from Teodor. Changes and notes:
* Instead of changing the order directly, now patch creates another patch with
modifier order of clauses. It does so for the normal sort as well as for
incremental sort. The whole thing is done in two steps: first it finds a
potentially better ordering taking into account number of groups, widths and
comparison costs; afterwards this information is used to produce a cost
estimation. This is implemented via a separate create_reordered_sort_path to
not introduce too many changes, I couldn't find any better place.
I haven't tested the patch with any queries, but I agree this seems like
the right approach in general.
I'm a bit worried about how complex the code in planner.c is getting -
the incremental sort patch already made it a bit too complex, and this
is just another step in that direction. I suppose we should refactor
add_paths_to_grouping_rel() by breaking it into smaller / more readable
pieces ...
* Function get_func_cost was removed at some point, but unfortunately this
patch was implemented before that, so it's still present there.
* For simplicity I've removed support in create_partial_grouping_paths, since
they were not covered by the existing tests anyway.
Hmmm, OK. I think that's something we'll need to address for the final
patch, but I agree we can add it after improving the costing etc.
* The costing part is pretty rudimentary and looks only at the first group.
It's mostly hand crafted to pass the existing tests.
OK, understood.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
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