PostgreSQL 7.1 forces sequence scan when there is no reason

Hello,

I have a problem with PostgreSQL 7.1 forces sequence scan which is
almost 10 times slower than index scan.

Here is the information about query, tables, and data.

Any advise would be nice.

db=> \d listmembers
                             Table "listmembers"
  Column   |  Type   |                       Modifiers
-----------+---------+-------------------------------------------------------
- id        | integer | not null default
 nextval('"listmembers_id_seq"'::text) server_id | integer |
 name      | text    |
 email     | text    |
Indexes: listmembers_sid_key
Unique keys: listmembers_id_key,
             listmembers_sid_email_key
db=> \d listmembers_sid_key
Index "listmembers_sid_key"
  Column   |  Type
-----------+---------
 server_id | integer
btree
db=> \d listmembers_sid_email_key
Index "listmembers_sid_email_key"
  Column   |  Type
-----------+---------
 server_id | integer
 email     | text
unique btree
db=> explain analyze select count(*) from listmembers where
 server_id = 15182; NOTICE:  QUERY PLAN:
Aggregate  (cost=31428.34..31428.34 rows=1 width=0) (actual
 time=38632.84..38632.84 rows=1 loops=1) ->  Seq Scan on listmembers
 (cost=0.00..31379.16 rows=19671 width=0) (actual time=198.13..38494.36
 rows=10011 loops=1) Total runtime: 38633.01 msec
EXPLAIN
db=> set enable_seqscan to no;
SET VARIABLE
db=> explain analyze select count(*) from listmembers where
 server_id = 15182; NOTICE:  QUERY PLAN:
Aggregate  (cost=60210.41..60210.41 rows=1 width=0) (actual
 time=2117.61..2117.61 rows=1 loops=1) ->  Index Scan using
 listmembers_sid_key on listmembers  (cost=0.00..60161.24 rows=19671 width=0)
 (actual time=81.41..2096.67 rows=10011 loops=1) Total runtime: 2117.83 msec
EXPLAIN
db=> select count(*) from listmembers where server_id = 15182;
 count
-------
 10011
(1 row)
db=> select count(*) from listmembers;
  count
---------
 1372425
(1 row)
db=> select server_id, count(*) from listmembers group by
 server_id order by count(*) desc limit 30; server_id | count
-----------+-------
     34062 | 43154
     32715 | 32232
     42495 | 31034
     38013 | 15905
     13746 | 15237
     42434 | 12045
     31696 | 10539
     15413 | 10334
     15182 | 10011
     28178 | 10000
     33330 | 10000
     37071 |  9960
     46532 |  9860
     15392 |  9737
     29643 |  9423
     36503 |  9329
     25378 |  9280
     32095 |  9079
     28621 |  9047
     15400 |  9043
     25870 |  8972
     38377 |  8724
     13046 |  8484
     42416 |  8442
     14869 |  7973
     24131 |  7940
     32625 |  7918
     46480 |  7783
     43172 |  7179
     36849 |  6887
(30 rows)
webmailstation=# select * from pg_class where relname='listmembers';
   relname   | reltype | relowner | relam | relfilenode | relpages |
 reltuples  | reltoastrelid | reltoastidxid | relhasindex | relisshared |
 relkind | relnatts | relchecks | reltriggers | relukeys | relfkeys | relrefs
 | relhasoids | relhaspkey | relhasrules | relhassubclass | relacl
 -------------+---------+----------+-------+-------------+----------+--------
-----+---------------+---------------+-------------+-------------+---------+-
---------+-----------+-------------+----------+----------+---------+---------
---+------------+-------------+----------------+-------- listmembers |
 6429403 |      102 |     0 |     6429402 |    14224 | 1.37241e+06 |
 6429404 |             0 | t           | f           | r       |        4 |
       0 |           0 |        0 |        0 |       0 | t          | f
    | f           | f              | (1 row)
db=# select * from pg_statistic where starelid=6429402 ;
 starelid | staattnum | stanullfrac | stawidth | stadistinct | stakind1 |
 stakind2 | stakind3 | stakind4 | staop1 | staop2 | staop3 | staop4 |
                                         stanumbers1
                         | stanumbers2 | stanumbers3  | stanumbers4 |

                              stavalues1

 |
                                 stavalues2
                                                                          |
 stavalues3 | stavalues4
 ----------+-----------+-------------+----------+-------------+----------+---
-------+----------+----------+--------+--------+--------+--------+-----------
-----------------------------------------------------------------------------
----------------------+-------------+--------------+-------------+-----------
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
---------------------------------------------------------------------------+-
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-----------------------------------------------------------------------+-----
-------+------------ 6429402 |         1 |           0 |        4 |
 -1 |        2 |        3 |        0 |        0 |     97 |     97 |      0 |
     0 |
                                         | {0.805365}  |              |
       |
 {590,520800,790589,1001533,1375234,1655946,1926816,2342644,2808910,3347435,3
532408}

         |


     |            | 6429402 |         2 |           0 |        4 |
 1150 |        1 |        2 |        3 |        0 |     96 |     97 |     97
 |      0 |
 {0.038,0.027,0.024,0.0143333,0.012,0.01,0.00933333,0.00766667,0.00733333,0.0
07}                              |             | {0.428932}   |             |
 {34062,32715,42495,15182,38013,36503,13746,46532,42434,15392}


         |
 {12855,15419,16223,20598,26019,30733,34402,38139,40811,44986,49509}

                                                                        |
        | 6429402 |         3 |   0.0376667 |       10 |        2581 |
 1 |        2 |        3 |        0 |     98 |    664 |    664 |      0 |
 {0.466333,0.00333333,0.003,0.002,0.002,0.00166667,0.00133333,0.00133333,0.00
1,0.001}                         |             | {0.227739}   |             |
 {"",webwizards,"The Ad Builder Safe List","Kane at InternetSeer","Network
 Commerce",Michael,James,John,Admin,"D.Woodward "}

            | {" Success Center","Brent Sommers",Debra,"Great
 Vacations","Johnny Blaze",Mariani,"Peter Maglione","Silhouettes
 Catalog",Wally,"johan kotze",жЛжН}
                                     |            | 6429402 |         4 |
       0 |       25 |   -0.118184 |        1 |        2 |        3 |        0
 |     98 |    664 |    664 |      0 |
 {0.00133333,0.001,0.001,0.000666667,0.000666667,0.000666667,0.000666667,0.00
0666667,0.000666667,0.000666667} |             | {-0.0167706} |             |
 {servicebox@spedia.net,dougsreplyto3@excite.com,inquire@careerexpansion.com,
234freeb@webwizards-add-url.com,cashdueu@hotmail.com,cashonline1@excite.com,c
wmailer@yahoo.com,galaxy@mail2.galaxy.com,gmichel@post.com,half.com_by_ebay@h
alf.com} |
 {05078475@email.com,bethebest@zwallet.com,cynric7@yahoo.com,ezine@yourhomejo
b.com,ilkst@beeline.samara.ru,kirk.stensrud@lpl.com,mjm@netset.com,ping13013@
yahoo.fr,sandrac@menta.net,tgaeke@worldsubmitter.com,zzzmuffin@aol.com} |
        | (4 rows)

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