Inconsistant use of index.

In porting a pretty large (10s of millions of records) data warehouse
from Oracle to PostgreSQL,

  Once some of my tables started getting pretty large, PostgreSQL
suddenly stopped using indexes when I use expressions like "col = value"
decreasing performance by 20X.  This meant that my daily reports started
taking two days instead of 2 hours to run!!!!).


  Interestingly when I re-write the queries using >= and <= to produce
identical results, the index works fine.  Example queries in question include:

   select count(*) from fact where dat='2002-03-01';
   select count(*) from fact where dat<='2002-03-01' and dat>='2002-03-01';

The distribution of values in "dat" are roughly evenly spaced from
'2002-01-01' through '2002-03-25'.

Attached below are

 A: Information about the table, including "\d" and "vacuum verbose analyze"
 B: Output of "explain analyze" from the above queries (showing the 20X
    slowdown)
 C: Version and configuration information.

Any suggestions on what I should look at next would be appreciated.

   Thanks much,
   Ron

PS: As a quite perverse workaround, I rewrote all my queries to have
   "col<=val and col>=val" everywhere I used to have "col=val"
   and everything is running fine again... but that's just wierd.





============================================================================
== A: Information about the table
============================================================================
logs2=# \d fact
                Table "fact"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 dat    | date                   |
 tim    | time without time zone |
 ip_id  | integer                |
 bid_id | integer                |
 req_id | integer                |
 ref_id | integer                |
Indexes: i_fact__bid_id,
         i_fact__dat,
         i_fact__ref_id,
         i_fact__req_id,
         i_fact__tim

logs2=# select count(*) from fact;
  count
----------
 18410778
(1 row)

logs2=# vacuum verbose analyze fact;
NOTICE:  --Relation fact--
NOTICE:  Pages 144967: Changed 0, Empty 0; Tup 18410778: Vac 0, Keep 0, UnUsed
0.
        Total CPU 11.56s/2.97u sec elapsed 71.91 sec.
NOTICE:  Analyzing fact
VACUUM


============================================================================
== B: Explain Analyze for the two queries.
==     Note that the <=, >= one was over 20X faster.
============================================================================

logs2=# explain analyze select count(*) from fact where dat='2002-03-01';
NOTICE:  QUERY PLAN:

Aggregate  (cost=375631.14..375631.14 rows=1 width=0) (actual
time=76689.42..76689.42 rows=1 loops=1)
  ->  Seq Scan on fact  (cost=0.00..375101.72 rows=211765 width=0) (actual
time=20330.96..76391.94 rows=180\
295 loops=1)
Total runtime: 76707.92 msec

EXPLAIN
logs2=# explain analyze select count(*) from fact where dat<='2002-03-01' and
                dat >='2002-03-01';
NOTICE:  QUERY PLAN:

Aggregate  (cost=5.98..5.98 rows=1 width=0) (actual time=2921.39..2921.39
rows=1 loops=1)
  ->  Index Scan using i_fact__dat on fact  (cost=0.00..5.98 rows=1 width=0)
(actual time=73.55..2583.53 ro\
ws=180295 loops=1)
Total runtime: 2921.55 msec

EXPLAIN
logs2=#





============================================================================
== C: Version and configuration information.
============================================================================

[17]localhost:~/apps/pgsql% psql -V
psql (PostgreSQL) 7.2
contains support for: readline, history
Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
Portions Copyright (c) 1996, Regents of the University of California
Read the file COPYRIGHT or use the command \copyright to see the
usage and distribution terms.

[17]localhost:/scratch/pgsql/data% diff -wiu postgresql.conf postgresql.conf.bak
--- postgresql.conf     Sat Mar 23 15:39:34 2002
+++ postgresql.conf.bak Tue Mar  5 19:33:54 2002
@@ -50,7 +50,7 @@
 #shared_buffers  = 10000        # 2*max_connections, min 16
 ##  goes to about 84 meg with 4000.
 #shared_buffers  = 4000        # 2*max_connections, min 16
-shared_buffers   = 10000        # 2*max_connections, min 16
+shared_buffers   = 8000        # 2*max_connections, min 16

 #max_fsm_relations = 100    # min 10, fsm is free space map
 #max_fsm_pages = 10000      # min 1000, fsm is free space map




--
   Ronald Mayer
   Director of Web Business
   InterVideo, Inc.