Date: Wed, 22 Nov 2017 08:57:13 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Artur =?utf-8?B?WmFqxIVj?= <azajac@ang.com.pl>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Bad estimates (DEFAULT_UNK_SEL)
Message-ID: <20171122145713.GU5668@telsasoft.com>
References: <03b401d3639e$5d7493a0$185dbae0$@ang.com.pl>
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On Wed, Nov 22, 2017 at 03:29:54PM +0100, Artur Zając wrote:
> CREATE TABLE xyz AS SELECT generate_series(1,10000000,1) AS gs;
> 
> db=# 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)
...
> newrr=# 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)

> I noticed that each additional clause reduces the number about 200 times and
> define DEFAULT_NUM_DISTINCT is responsible for this behaviur.

I think it's actually:

src/include/utils/selfuncs.h-/* default selectivity estimate for boolean and null test nodes */
src/include/utils/selfuncs.h-#define DEFAULT_UNK_SEL                    0.005

..which is 1/200.

Note, you can do this, which helps a bit by collecting stats for the index
expr:

postgres=# CREATE INDEX ON xyz((gs&1));
postgres=# ANALYZE xyz;
postgres=# explain analyze SELECT * FROM xyz WHERE gs&1=1 AND gs&2=2 AND gs&4=4;
 Bitmap Heap Scan on xyz  (cost=91643.59..259941.99 rows=124 width=4) (actual time=472.376..2294.035 rows=1250000 loops=1)
   Recheck Cond: ((gs & 1) = 1)
   Filter: (((gs & 2) = 2) AND ((gs & 4) = 4))
   Rows Removed by Filter: 3750000
   Heap Blocks: exact=44248
   ->  Bitmap Index Scan on xyz_expr_idx  (cost=0.00..91643.55 rows=4962016 width=0) (actual time=463.477..463.477 rows=5000000 loops=1)
         Index Cond: ((gs & 1) = 1)

Justin