bad select performance for where (x=1 or x=3) - Mailing list pgsql-sql

[PostgreSQL 6.5.0 on i586-pc-linux-gnu, compiled by gcc egcs-2.91.66]

I have a 'select' including 'where (x=1 or x=3)' that takes 16 times as long
as with just 'where x=1'.  Here's a (somewhat simplified) example:

table opset_steps
      (name text, id int2, ver int2) [1400 rows]
      non-unique index is on (id, ver)

table run_opsets
      (status int2, id int2, ver int2, run_id int2, seq int2) [17000 rows]
      pkey is (id, seq), second index on(status, id, ver, run_id)
      select count(*) from run_opsets where status=1; --> 187
      select count(*) from run_opsets where status=3; --> 10564

table runs
      (run_name text, run_id int2, status int2) [900 rows]
      pkey is run_name, second index(run_id, status)

I have vacuum analyzed all relevant tables.

This query takes 16 seconds (without the explain of course):
  explain select os.name,r.run_name,ro.status from opset_steps os,runs r,run_opsets ro where (ro.status=1 or
ro.status=3)and ro.opset_id=os.opset_id and ro.run_id=r.run_id and ro.opset_ver=os.opset_ver and r.status=1; 

Hash Join  (cost=1793.58 rows=14560 width=38)
  ->  Hash Join  (cost=1266.98 rows=14086 width=24)
        ->  Seq Scan on run_opsets ro  (cost=685.51 rows=13903 width=8)
        ->  Hash  (cost=70.84 rows=1389 width=16)
              ->  Seq Scan on opset_steps os  (cost=70.84 rows=1389 width=16)
  ->  Hash  (cost=47.43 rows=374 width=14)
        ->  Seq Scan on runs r  (cost=47.43 rows=374 width=14)


This query takes just under one second:[diff is status=1 instead of (1 or 3)]
  explain select os.name,r.run_name,ro.status from opset_steps os,runs r,run_opsets ro where ro.status=1 and
ro.opset_id=os.opset_idand ro.run_id=r.run_id and ro.opset_ver=os.opset_ver and r.status=1; 

Hash Join  (cost=1359.57 rows=7719 width=38)
  ->  Hash Join  (cost=1051.39 rows=7467 width=24)
        ->  Seq Scan on run_opsets ro  (cost=685.51 rows=7370 width=8)
        ->  Hash  (cost=70.84 rows=1389 width=16)
              ->  Seq Scan on opset_steps os  (cost=70.84 rows=1389 width=16)
  ->  Hash  (cost=47.43 rows=374 width=14)
        ->  Seq Scan on runs r  (cost=47.43 rows=374 width=14)


Why should it take over 16 times as long for (status=1 or 3) as for status=1?
I have indexes on exactly all the fields used in the where clause.
I tried "status in (1,3)" with no improvement.


George Young,  Rm. L-204        gry@ll.mit.edu
MIT Lincoln Laboratory
244 Wood St.
Lexington, Massachusetts  02420-9108    (781) 981-2756