From: =?iso-8859-2?Q?Artur_Zaj=B1c?= <azajac@ang.com.pl>
To: <pgsql-performance@lists.postgresql.org>
Subject: Bad estimates
Date: Wed, 22 Nov 2017 15:29:54 +0100
Message-ID: <03b401d3639e$5d7493a0$185dbae0$@ang.com.pl>
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-2"
Content-Transfer-Encoding: 7bit
Content-Language: pl
Thread-Index: AdNjmY4BO/1NbvVZQ9iUO+V730y+Rw==
Precedence: bulk

Hi,

We have table created like this:

CREATE TABLE xyz AS SELECT generate_series(1,10000000,1) AS gs;

Now:

db=# explain analyze select * from xyz where gs&1=1;

                                                  QUERY PLAN
----------------------------------------------------------------------------
-----------------------------------
 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
 Planning time: 0.133 ms
 Execution time: 3340.886 ms
(5 rows)

And after adding additional clause to WHERE:

db=# explain analyze select * from xyz where gs&1=1 and gs&2=2;

                                                 QUERY PLAN
----------------------------------------------------------------------------
---------------------------------
 Seq Scan on xyz  (cost=0.00..329735.50 rows=345 width=4) (actual
time=0.045..3010.430 rows=2500000 loops=1)
   Filter: (((gs & 1) = 1) AND ((gs & 2) = 2))
   Rows Removed by Filter: 7500000
 Planning time: 0.106 ms
 Execution time: 3176.355 ms
(5 rows)

And one more clause:

newrr=# explain analyze select * from xyz where gs&1=1 and gs&2=2 and
gs&4=4;
                                                QUERY PLAN
----------------------------------------------------------------------------
-------------------------------
 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
 Planning time: 0.119 ms
 Execution time: 3415.839 ms
(5 rows)

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.

Artur Zajac