| From: | Francisco Olarte <folarte(at)peoplecall(dot)com> |
|---|---|
| To: | Kaijiang Chen <chenkaijiang(at)gmail(dot)com> |
| Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Pg Bugs <pgsql-bugs(at)postgresql(dot)org> |
| Subject: | Re: BUG #14302: SQL with LIMIT degrades performance seriously |
| Date: | 2016-08-31 13:55:30 |
| Message-ID: | CA+bJJbyuQ2t4Q_2iaGuY9QvLnB+vDJ8C9LV7T01kASPA28jbsA@mail.gmail.com |
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| Lists: | pgsql-bugs |
On Wed, Aug 31, 2016 at 6:09 AM, Kaijiang Chen <chenkaijiang(at)gmail(dot)com> wrote:
> In the sql "select * from renren.user_relations where parent_id=846346 order
> by user_id LIMIT 10;", planner can query the number of rows (with
> parent_id=846346) from the btree index, right?
> If so, planner will know that it's better to sort the rows (actually less
> than 2000) than to scan 10M rows.
mmmm, AFAIK planner does NOT query, it plans using stored statistics.
This is why some data distribution, which are a bad match for the
postgres ways of storing stats, tend to plan ( oacasionally ) badly.
Francisco Olarte.
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