Hi Harada-san,
Thank you for the review. I think that the parse tree has enough information to do this optimization and that the easiest way to do it is to use the information, though I might not have understand your comments correctly. So, I would like to fix the bug by simply modifying the removability check in adjust_targetlist() so that the resjunk column is not used in GROUP BY, DISTINCT ON and *window PARTITION/ORDER BY*, besides ORDER BY. No? I am open to any comments. Thanks, Best regards, Etsuro Fujita From: Hitoshi Harada [mailto:umi.tan...@gmail.com] Sent: Wednesday, June 19, 2013 2:57 PM To: Etsuro Fujita Cc: Tom Lane; Alexander Korotkov; pgsql-hackers Subject: Re: [HACKERS] Patch for removng unused targets On Tue, Jun 18, 2013 at 5:15 AM, Etsuro Fujita <fujita.ets...@lab.ntt.co.jp> wrote: Hi Alexander, I wrote: > > > From: Tom Lane [mailto:t...@sss.pgh.pa.us] > > > > resjunk means that the target is not supposed to be output by the query. > > > Since it's there at all, it's presumably referenced by ORDER BY or GROUP > > > BY or DISTINCT ON, but the meaning of the flag doesn't depend on that. > > > > What you would need to do is verify that the target is resjunk and not > > > used in any clause besides ORDER BY. I have not read your patch, but > > > I rather imagine that what you've got now is that the parser checks this > > > and sets the new flag for consumption far downstream. Why not just make > > > the same check in the planner? > > > I've created a patch using this approach. > > I've rebased the above patch against the latest head. Could you review the > patch? If you have no objection, I'd like to mark the patch "ready for > committer". Sorry, I've had a cleanup of the patch. Please find attached the patch. Don't forget about window functions! test=# EXPLAIN (ANALYZE, VERBOSE) SELECT *, count(*) over (partition by slow_func(x,y)) FROM test ORDER BY slow_func(x,y) LIMIT 10; QUERY PLAN -------------------------------------------------------------------------------- ----------------------------------------------------------- Limit (cost=0.28..3.52 rows=10 width=16) (actual time=20.860..113.764 rows=10 loops=1) Output: x, y, (count(*) OVER (?)) -> WindowAgg (cost=0.28..324.27 rows=1000 width=16) (actual time=20.858..113.747 rows=10 loops=1) Output: x, y, count(*) OVER (?) -> Index Scan using test_idx on public.test (cost=0.28..59.27 rows=1000 width=16) (actual time=10.563..113.530 rows=11 loops=1) Output: slow_func(x, y), x, y Total runtime: 117.889 ms (7 rows) And I don't think it's a good idea to rely on the parse tree to see if we can remove those unused columns from the target list, because there should be a lot of optimization that has been done through grouping_planner, and the parse tree is not necessarily representing the corresponding elements at this point. I think it'd be better to see path keys to find out the list of elements that may be removed, rather than SortClause, which would be a more generalized approach. Thanks, -- Hitoshi Harada
