Thanks Dmitry for reply.
Backporting optimization from 2.5 to 2.1 would be a big work ?
L.

> >
> > I guess, that it is because COALESCE is internaly transformed into CASE
> > WHEN <subselect> IS NULL then 0 else <subselect> end
> > So in expression is subselect twice.
>
> Correct.
>
> > My question is if <subselect> is really executed twice for every row in
> > table1? or is there internal optimization where duplicate expressions
> > are reduced?
>
> Optimization happens only in Firebird 2.5 and above:
> http://tracker.firebirdsql.org/browse/CORE-501
>
>
> Dmitry
>