Re: Bad query? Or planner? - Mailing list pgsql-general

From Devin Smith
Subject Re: Bad query? Or planner?
Date
Msg-id CAMfqxKFQp0fX_tDKi+xQvbYdyX6u1LTRrdiphWpUYSxxdwyziw@mail.gmail.com
Whole thread Raw
In response to Re: Bad query? Or planner?  (Johann Spies <johann.spies@gmail.com>)
List pgsql-general
Correct. There is no WHERE filter in the first query. The JOIN condition from the first query was moved into a WHERE filter in the second query (enabled by LATERAL). Both have the same ordering applied with a limit of 1.

I chatted with a couple users in the IRC channel, and I think I got the consensus that the two queries are semantically the same, but the query planner doesn't currently optimize my original query.

-Devin

On Mon, Dec 5, 2016 at 1:59 AM Johann Spies <johann.spies@gmail.com> wrote:


On 28 November 2016 at 21:11, Devin Smith <dsmith@redcurrent.com> wrote:
Hi,


I recently wrote a query that I thought was easy to reason about, and I assumed the query planner would execute it efficiently.

SELECT * FROM xtag_stack_feed
JOIN (
  SELECT DISTINCT ON (do_post_xtag.xtag_ci) * 
  FROM do_post_xtag
  JOIN do_post ON do_post.rc_id=do_post_xtag.post_rc_id
  ORDER BY do_post_xtag.xtag_ci ASC, do_post_xtag.post_rc_id DESC) last_post
ON last_post.xtag_ci=xtag_stack_feed.xtag_ci
ORDER BY decayed_to_base DESC
LIMIT 1;

Unfortunately, the query as written is not being executed efficiently. I tried to rewrite it in a couple different ways without success, and then learned about lateral joins. Rewritten as follows, it executes efficiently.

SELECT * FROM xtag_stack_feed
JOIN LATERAL (
  SELECT DISTINCT ON (do_post_xtag.xtag_ci) *
  FROM do_post_xtag
  JOIN do_post ON do_post.rc_id=do_post_xtag.post_rc_id
  WHERE do_post_xtag.xtag_ci=xtag_stack_feed.xtag_ci
  ORDER BY do_post_xtag.xtag_ci ASC, do_post_xtag.post_rc_id DESC) last_post
ON true
ORDER BY decayed_to_base DESC
LIMIT 1;

From my naive perspective, it seems like the second query is semantically equivalent to the first; it just has the join condition moved into the subquery as a WHERE filter.



I do not see a "where"  condition in your first query.

Regards
Johann


--
Because experiencing your loyal love is better than life itself,
my lips will praise you.  (Psalm 63:3)

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