Re: Bad query? Or planner?

On 28 November 2016 at 21:11, Devin Smith <dsmith(at)redcurrent(dot)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

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