Index problem

I'm totally baffled as to why postgres won't use this index. I have a
table "point" with an index on it as so:

CREATE INDEX unit_point
   ON point
   USING btree
   (unit_id, time);

When I try the following query, it insists on using a seqscan:

SELECT time FROM point
WHERE unit_id = 95501 and flags & 64 = 64
ORDER BY time DESC LIMIT 1;

If I enable_seqscan to off, then it uses the index. I have timed both
options and using the index is about 3 times faster. I have run vacuum
analyze on this table.  Here is the explain output:

seqscan = on:
Limit  (cost=12883.87..12883.87 rows=1 width=8)
   ->  Sort  (cost=12883.87..12883.99 rows=46 width=8)
         Sort Key: "time"
         ->  Seq Scan on point  (cost=0.00..12882.60 rows=46 width=8)
               Filter: ((unit_id = 95501::oid) AND (((flags)::integer &
64) = 64))

seqscan = off:
Limit  (cost=19710.90..19710.90 rows=1 width=8)
   ->  Sort  (cost=19710.90..19711.01 rows=46 width=8)
         Sort Key: "time"
         ->  Index Scan using unit_point on point  (cost=0.00..19709.63
rows=46 width=8)
               Index Cond: (unit_id = 95501::oid)
               Filter: (((flags)::integer & 64) = 64)

Can somebody please help?
--
David Mitchell
Software Engineer
Telogis

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