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: 5000000Planning time: 0.133
msExecutiontime: 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:
7500000Planningtime: 0.106 msExecution 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
Removedby Filter: 8750000Planning time: 0.119 msExecution 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