On Sun, 1 Feb 2004 21:07:59 +0000 (UTC), David Johnson wrote:

>> ...
>
> Hmmm ... so Firebird does not support index-only scans. That is
> interesting and important to know. Index only scans can result in orders
> of improvement in performance in some queries. It does not impact my
> immediate project, but I have seen the addition of an index for the sole
> purpose of acquiring index only scans on other DBMS's improve performance
> of a complex operation from 30 minutes (after prepare) to 10 seconds
> (after prepare). The actual prepare took 10 minutes in both with and
> without the additional index, but using precompiled and static SQL it
> never impacted my users. It looks like I need to write a couple more
> tests. :o)

One reason for not being able to use a 'pure index scan' is that Fb has to
look at the records itself to determine whether your transaction really can
'see' this record.
AFAIU, this is also the reason why a count(*) has to look at all the
records - i.e. doing a table scan.


--
Aage J.