What can I do to speed up queries like the following
select count(*) from ttt group by xxx;
Using Postgres 7.1 on Solaris 8 the above query on a 1M row
database takes 6 times longer than the following
select count(*) from ttt;
With Postgres "group by" is apparently quite expensive. Using
Oracle and MySQL the same query and the same data, the first query
takes only 25% longer. I think both of these DBMSes use a single
sequential scan of the data while Postgresql (using the explain query)
uses a multi pass process. Yes I did a vacuum analyze.
Is there anything I as user can do. Build some kind of index?
In absolut times "group by" is a killer. All querries on a single
1M row table that include "group by" take about 3 minutes. Oracle
takes about 40 seconds and MySQL about 25 seconds.
Here is what EXPLAIN shows.
alberch=# explain select count(*) from tyc_main group by nphoto;
NOTICE: QUERY PLAN:
Aggregate (cost=170404.22..175695.88 rows=105833 width=2)
-> Group (cost=170404.22..173050.05 rows=1058332 width=2)
-> Sort (cost=170404.22..170404.22 rows=1058332 width=2)
-> Seq Scan on tyc_main (cost=0.00..49705.32 rows=1058332
width=2)
EXPLAIN
alberch=# explain select count(*) from tyc_main;
NOTICE: QUERY PLAN:
Aggregate (cost=52351.15..52351.15 rows=1 width=0)
-> Seq Scan on tyc_main (cost=0.00..49705.32 rows=1058332 width=0)
EXPLAIN
When I run the queries the last takes 18 sec, the first
a little over 5 _minutes_
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