Message-ID: <1511363394.2553.68.camel@cybertec.at>
Subject: Re: Bad estimates
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Artur =?UTF-8?Q?Zaj=C4=85c?= <azajac@ang.com.pl>,
	pgsql-performance@lists.postgresql.org
Date: Wed, 22 Nov 2017 16:09:54 +0100
In-Reply-To: <03b401d3639e$5d7493a0$185dbae0$@ang.com.pl>
References: <03b401d3639e$5d7493a0$185dbae0$@ang.com.pl>
Organization: Cybertec
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Artur Zając wrote:
> We have table created like this:
> 
> CREATE TABLE xyz AS SELECT generate_series(1,10000000,1) AS gs;
> 
> Now:
> 
> explain analyze select * from xyz where gs&1=1;

>  Seq Scan on xyz  (cost=0.00..260815.38 rows=68920 width=4)
>                   (actual time=0.044..2959.728 rows=5000000 loops=1)
>    Filter: ((gs & 1) = 1)
>    Rows Removed by Filter: 5000000
[...]
> And one more clause:
> 
> explain analyze select * from xyz where gs&1=1 and gs&2=2 and gs&4=4;

>  Seq Scan on xyz  (cost=0.00..398655.62 rows=2 width=4)
>                   (actual time=0.052..3329.422 rows=1250000 loops=1)
>    Filter: (((gs & 1) = 1) AND ((gs & 2) = 2) AND ((gs & 4) = 4))
>    Rows Removed by Filter: 8750000

> As we can see estimates differs significally from the actual records count -
> only three clauses are reducing estimated number of records from 10000000 to
> 2.
> 
> I noticed that each additional clause reduces the number about 200 times and
> define DEFAULT_NUM_DISTINCT is responsible for this behaviur.
> 
> I think that this variable should be lower or maybe estimation using
> DEFAULT_NUM_DISTTINCT should be done once per table.

The problem is that the expression "gs & 1" is a black box for the
optimizer; it cannot estimate how selective the condition is and falls
back to a default value that is too low.

You can create an index to
a) improve the estimate
and
b) speed up the queries:

CREATE INDEX ON xyz ((gs & 1), (gs & 2), (gs & 4));

Don't forget to ANALYZE afterwards.

Yours,
Laurenz Albe