| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | alexey(dot)ermakov(at)dataegret(dot)com |
| Cc: | pgsql-bugs(at)lists(dot)postgresql(dot)org |
| Subject: | Re: BUG #15160: planner overestimates number of rows in join when there are more than 200 rows coming from CTE |
| Date: | 2018-04-17 14:15:38 |
| Message-ID: | 15035.1523974538@sss.pgh.pa.us |
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| Lists: | pgsql-bugs pgsql-hackers |
=?utf-8?q?PG_Bug_reporting_form?= <noreply(at)postgresql(dot)org> writes:
> I'm wondering how planner estimates number of rows in that case:
See eqjoinsel_semi, particularly the change in behavior when it thinks
nd2 is or is not a default estimate.
Given the lack of statistics about the output of the WITH clause,
it's hard to see how we'd ever get trustworthy estimates here.
I think the fact that your first example yields an accurate
estimate is mostly luck.
regards, tom lane
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