Xavier,

X> Yes, but how can I know which is the best plan for a query? (I mean "a
X> priori", without doing try-and-see the performance) That's the sort of
X> things I would like to find in the docs I'm looking for.

Get hold of IBPLANalyzer (find it on Borland's CodeCentral). The tool
may not work with your version of Firebird, but the documentation
applies and it includes a thorough tutorial on query optimization.

Ciao
--
Nando mailto:nandod@...