sorry - I am correcting myself - I spoke about 'order by', not 'group
by'.

--- In firebird-support@yahoogroups.com, "vincent_kwinsey"
<vincent_kwinsey@...> wrote:

>
> Hello!
>
> I am trying to find optimal select statement. Currently I have only
> indexed reads (no non-indexed reads), but I am still trying to

minimize

> this number - on some test cases I can see improvement some 10

times -

> for the number of reads and execution time as well.
>
> There are strange things:
> - adding 'group by 1, 2, 3' clause removes all the optimization and
> there is max number of reads
> - 'union' remove all the optimization as opposite to 'union all'.

In

> case of 'union' the total number of reads excees the sum of reads

for

> individual selects (which are parts or 'union').
>
> Intuition says than in both situations there should be some fixed
> amount of reads and the 'group' or 'union' should simply make some
> sorting or elimination of duplications of some temporary set and

there

> should be no additional reads...
>