Re: no index-usage on aggregate-functions?

> 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

This cannot be made O(1) in postgres due to MVCC.  You just have to live
with it.

> 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

Average and sum can never use an index AFAIK, in any db server.  You
need information from every row.

> SELECT Max(date_) AS LatestDate FROM the_table --(index on date_)
> => Seq scan -> takes about 14 sec

Yep, that's due to postgresql's type extensibility.  You should use th
workaround you point out below.

> 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?

Nope.

Chris