| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
| Cc: | postgresql(at)foo(dot)me(dot)uk, postgres performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Slow query: bitmap scan troubles |
| Date: | 2012-12-06 17:27:48 |
| Message-ID: | CAMkU=1xXdiB+chgqwzWoN+7qqag9DeMTtDA1HWQYvuot6yDyJg@mail.gmail.com |
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| Lists: | pgsql-hackers pgsql-performance |
On Wed, Dec 5, 2012 at 9:43 AM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
> On Wed, Dec 5, 2012 at 2:39 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> I'm not sure that this change would fix your problem, because it might
>> also change the costs of the alternative plans in a way that
>> neutralizes things. But I suspect it would fix it. Of course, a
>> correct estimate of the join size would also fix it--you have kind of
>> a perfect storm here.
>
> As far as I can see on the explain, the misestimation is 3x~4x not 200x.
It is 3x (14085 vs 4588) for selectivity on one of the tables, "Index
Only Scan using idx_trade_id_book on trade".
But for the join of both tables it is estimate 2120 vs actual 11.
Cheers,
Jeff
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