Hi,
I've experienced that PG up to current release does not make use of an index when aggregating. Which of course may
resultin unacceptable answering times
This behaviour is reproducable on any table with any aggregat function in all of my databases on every machine
(PostgreSQL7.4.2 on i386-redhat-linux-gnu and PostgreSQL 7.2.1 on i686-pc-linux-gnu)
f.e. querying against a 2.8-mio-records (2.800.000) table the_table
SELECT count(*) FROM the_table
=> Seq scan -> takes about 12 sec
SELECT Avg(num_found) AS NumFound FROM the_table --(index on num_found)
=> Seq scan -> takes about 10 sec
SELECT Sum(num_found) AS TotalFound FROM the_table --(index on num_found)
=> Seq scan -> takes about 11 sec
SELECT Max(date_) AS LatestDate FROM the_table --(index on date_)
=> Seq scan -> takes about 14 sec
But
SELECT date_ AS LatestDate FROM the_table ORDER BY date_ DESC LIMIT 1;
=> Index scan -> takes 0.18 msec
MS SQLServer 2000: Use of an appropriate index _whenever_ aggregating.
Am I doing something wrong?
Greetings Harald
