seems like i kicked off some real long thread with this question :-)

--- In firebird-support@yahoogroups.com, Dmitry Yemanov <dimitr@...>
wrote:

>
> Leyne, Sean wrote:
> >
> > Recursion??? There is no recursion.
> >
> > The engine does not treat the derived table like an IN (SELECT ...
> >
> > Rather it resolves the inner result set and then passes it to the next
> > level for operation.
>
> In fact, it depends on the generated plan. We almost never materialize
> the result sets, so unless SORT MERGE is in game, the derived table is
> evaluated in a nested loop. However, it benefits from a proper indexing
> much and hence it's not a big deal in practice.
>
> > As for performance, the statement:
> >
> > SELECT * FROM (SELECT FIRST 5 * FROM Table ORDER BY 1 DESC) ORDER BY 1
> >
> > Executes without any perceptible time difference, as compared to:
> >
> > SELECT FIRST 5 * FROM Table ORDER BY 1 DESC
>
> Not a surprise at all :-)
>
>
> Dmitry
>