Re: [PERFORM] select max(id) from aTable is very slow

[Nick Barr]

Nick Barr wrote:

David Teran wrote:

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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Oops that should be

SELECT id FROM theTable ORDER BY id DESC LIMIT 1;

Nick





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