Hi,
we have a table with about 6.000.000 rows. There is an index on a column with the name id which is an integer and serves as primary key.
When we execute select max(id) from theTable; it takes about 10 seconds. Explain analyze returns:
------------------------------------------------------------------------ --------------------------------------------------------
Aggregate (cost=153635.15..153635.15 rows=1 width=4) (actual time=9738.263..9738.264 rows=1 loops=1)
-> Seq Scan on job_property (cost=0.00..137667.32 rows=6387132 width=4) (actual time=0.102..7303.649 rows=6387132 loops=1)
Total runtime: 9738.362 ms
(3 rows)
I recreated the index on column id and ran vacuum analyze job_property but this did not help. I tried to force index usage with SET ENABLE_SEQSCAN TO OFF; but the explain analyze still looks like the query is done using a seqscan.
Is the speed more or less normal for a 'dual G5 with 2 GHZ and 4 GB of Ram and a SATA hd' or do i miss something?
regards David
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Try using:
SELECT id FROM theTable ORDER BY is DESC LIMIT 1;
Using COUNT, MAX, MIN and any aggregate function on the table of that size will always result in a sequential scan. There is currently no way around it although there are a few work arounds. See the following for more information.
http://archives.postgresql.org/pgsql-performance/2004-01/msg00045.php http://archives.postgresql.org/pgsql-performance/2004-01/msg00054.php http://archives.postgresql.org/pgsql-performance/2004-01/msg00059.php
HTH
Nick
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