Re: BUG #15233: Error in estimation leads to very bad parralel planin simple 2 table join.

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От Maxim Boguk
Тема Re: BUG #15233: Error in estimation leads to very bad parralel planin simple 2 table join.
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Msg-id CAK-MWwRh=J70OTJYtct-j=RV2BmPtq4bhMzgPtwN4Vc0ygBcxQ@mail.gmail.com
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Ответ на Re: BUG #15233: Error in estimation leads to very bad parralel planin simple 2 table join.  (Amit Kapila <amit.kapila16@gmail.com>)
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>  Planning time: 2.953 ms
>  Execution time: 2084537.882 ms
>
> Problem with estimation of selectivity
>    ->  Seq Scan on area a  (cost=0.00..265.70 rows=1 width=106) (actual
> time=0.669..1.756 rows=23 loops=1)
>         
​​
Filter: ((path)::text ~~ '%.1806.%'::text)
> leads to very dangerous idea to perform nested loop with very heavy parallel
> plan inside.
>

Is it because you have not performed Analyze on the 'area' or is it
something else due to which there is such a deviation in estimation?



​Hi,​

I don't think there is good way estimate selectivity of
​
Filter: ((path)::text ~~ '%.1806.%'::text)

But problem that there huge difference in real plan cost with exactly 1 row vs 2 or more rows estimated. Problem still exists without parallel plan involved, where is leads to 23 seq scan over table with 100M rows :(.


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Maxim Boguk
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