Strangely Variable Query Performance

Hey there;

On a Postgres 8.2.3 server, I've got a query that is running very slow in
some cases. With some work, I've determined the 'slow part' of the query.
:) This is a query on a table with like 10 million rows or something like
that. encounter_id is an integer and receipt is of type 'date'.

This query runs really slow [minutes] (query and explain below):

select extract(epoch from ds.receipt) from detail_summary ds where
ds.receipt >= '1998-12-30 0:0:0' and
ds.encounter_id in
(8813186,8813187,8813188,8813189,8813190,8813191,8813192,
8813193,8813194,8813195,8813196,8813197,8813198,8813199,
8813200,8813201,8813202,8813203,8813204,8813205,8813206,
8813207,8813208,8813209,8813210,8813211,8813212,8813213,
8813214,8813215,8813216,8813217,8813218,8813219,8813220,
8813221,8813222,8813223,8813224,8813225,8813226,8813227,
8813228,8813229,8813230,8813231,8813232,8813233,8813234,
8813235,8813236,8813237,8813238,8813239,8813240,8813241,
8813242,8813243,8813244,8813245,8813246,8813247,8813248,
8813249,8813250,8813251,8813252,8813253,8813254,8813255,
8813256,8813257,8813258,8813259,8813260,8813261,8813262,
8813263,8813264,8813265,8813266,8813267,8813268,8813269,
8813270,8813271,8813272,8813273,8813274,8813275,8813276,
8813277,8813278,8813279,8813280,8813281,8813282,8813283,
8813284,8815534)

Results in the 'explain' :

Seq Scan on detail_summary ds (cost=0.00..1902749.83 rows=9962 width=4)
Filter: ((receipt >= '1998-12-30'::date) AND (encounter_id = ANY
('{8813186,8813187,8813188,8813189,8813190,8813191,8813192,8813193,8813194,8813195,8813196,8813197,8813198,8813199,8813200,8813201,8813202,8813203,8813204,8813205,8813206,8813207,8813208,8813209,8813210,8813211,8813212,8813213,8813214,8813215,8813216,8813217,8813218,8813219,8813220,8813221,8813222,8813223,8813224,8813225,8813226,8813227,8813228,8813229,8813230,8813231,8813232,8813233,8813234,8813235,8813236,8813237,8813238,8813239,8813240,8813241,8813242,8813243,8813244,8813245,8813246,8813247,8813248,8813249,8813250,8813251,8813252,8813253,8813254,8813255,8813256,8813257,8813258,8813259,8813260,8813261,8813262,8813263,8813264,8813265,8813266,8813267,8813268,8813269,8813270,8813271,8813272,8813273,8813274,8813275,8813276,8813277,8813278,8813279,8813280,8813281,8813282,8813283,8813284,8815534}'::integer[])))
(2 rows)

Turning enable_seqscan to off results in a slightly more interesting
explain, but an equally slow query.

HOWEVER! The simple removal of the receipt date paramater results in a
fast query, as such:

select extract(epoch from ds.receipt) from detail_summary ds where
ds.encounter_id in
(8813186,8813187,8813188,8813189,8813190,8813191,8813192,
8813193,8813194,8813195,8813196,8813197,8813198,8813199,
8813200,8813201,8813202,8813203,8813204,8813205,8813206,
8813207,8813208,8813209,8813210,8813211,8813212,8813213,
8813214,8813215,8813216,8813217,8813218,8813219,8813220,
8813221,8813222,8813223,8813224,8813225,8813226,8813227,
8813228,8813229,8813230,8813231,8813232,8813233,8813234,
8813235,8813236,8813237,8813238,8813239,8813240,8813241,
8813242,8813243,8813244,8813245,8813246,8813247,8813248,
8813249,8813250,8813251,8813252,8813253,8813254,8813255,
8813256,8813257,8813258,8813259,8813260,8813261,8813262,
8813263,8813264,8813265,8813266,8813267,8813268,8813269,
8813270,8813271,8813272,8813273,8813274,8813275,8813276,
8813277,8813278,8813279,8813280,8813281,8813282,8813283,
8813284 ,8815534)

This query returns instantly and explains as:

Bitmap Heap Scan on detail_summary ds (cost=161.00..14819.81 rows=9963
width=4)
Recheck Cond: (encounter_id = ANY
('{8813186,8813187,8813188,8813189,8813190,8813191,8813192,8813193,8813194,8813195,8813196,8813197,8813198,8813199,8813200,8813201,8813202,8813203,8813204,8813205,8813206,8813207,8813208,8813209,8813210,8813211,8813212,8813213,8813214,8813215,8813216,8813217,8813218,8813219,8813220,8813221,8813222,8813223,8813224,8813225,8813226,8813227,8813228,8813229,8813230,8813231,8813232,8813233,8813234,8813235,8813236,8813237,8813238,8813239,8813240,8813241,8813242,8813243,8813244,8813245,8813246,8813247,8813248,8813249,8813250,8813251,8813252,8813253,8813254,8813255,8813256,8813257,8813258,8813259,8813260,8813261,8813262,8813263,8813264,8813265,8813266,8813267,8813268,8813269,8813270,8813271,8813272,8813273,8813274,8813275,8813276,8813277,8813278,8813279,8813280,8813281,8813282,8813283,8813284,8815534}'::integer[]))
-> Bitmap Index Scan on detail_summary_encounter_id_idx
(cost=0.00..160.75 rows=9963 width=0)
Index Cond: (encounter_id = ANY
('{8813186,8813187,8813188,8813189,8813190,8813191,8813192,8813193,8813194,8813195,8813196,8813197,8813198,8813199,8813200,8813201,8813202,8813203,8813204,8813205,8813206,8813207,8813208,8813209,8813210,8813211,8813212,8813213,8813214,8813215,8813216,8813217,8813218,8813219,8813220,8813221,8813222,8813223,8813224,8813225,8813226,8813227,8813228,8813229,8813230,8813231,8813232,8813233,8813234,8813235,8813236,8813237,8813238,8813239,8813240,8813241,8813242,8813243,8813244,8813245,8813246,8813247,8813248,8813249,8813250,8813251,8813252,8813253,8813254,8813255,8813256,8813257,8813258,8813259,8813260,8813261,8813262,8813263,8813264,8813265,8813266,8813267,8813268,8813269,8813270,8813271,8813272,8813273,8813274,8813275,8813276,8813277,8813278,8813279,8813280,8813281,8813282,8813283,8813284,8815534}'::integer[]))

Any thoughts? Both encounter_id and receipt date are indexed columns.
I've vacuumed and analyzed the table. I tried making a combined index of
encounter_id and receipt and it hasn't worked out any better.

Thanks!

Steve