Андрей Репко wrote:
RH> What happens if you use something like
RH> SELECT DISTINCT alias_id FROM ma_data;
sart_ma=# EXPLAIN ANALYZE SELECT DISTINCT alias_id FROM ma_data;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Unique (cost=65262.63..66770.75 rows=32 width=4) (actual
time=16780.214..18250.761 rows=32 loops=1)
-> Sort (cost=65262.63..66016.69 rows=301624 width=4) (actual
time=16780.204..17255.129 rows=301624 loops=1)
Sort Key: alias_id
-> Seq Scan on ma_data (cost=0.00..37811.24 rows=301624 width=4)
(actual time=6.896..15321.023 rows=301624 loops=1)
Total runtime: 18292.542 ms
sart_ma=# EXPLAIN ANALYZE SELECT alias_id FROM ma_data GROUP BY alias_id;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=38565.30..38565.62 rows=32 width=4) (actual
time=15990.863..15990.933 rows=32 loops=1)
-> Seq Scan on ma_data (cost=0.00..37811.24 rows=301624 width=4) (actual
time=3.446..14572.141 rows=301624 loops=1)
Total runtime: 15991.244 ms
OK - the planner thinks it's doing the right thing, your cost estimates
are way off. If you look back at where you got an index-scan, it's cost
was 1.1 million.
Index Scan using reference_9_fk on ma_data (cost=0.00..1139526.57
That's way above the numbers for seq-scan+hash/sort, so if the cost
estimate was right PG would be making the right choice. Looks like you
need to check your configuration settings. Have you read:
http://www.powerpostgresql.com/PerfList
or
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html
--
Richard Huxton
Archonet Ltd
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