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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