> What happen if all fields in WHERE, are indexes? which of them is
> more important?

Firebird ranks usefulness by the "selectivity" of the index. That is
pretty much how unique is each value in the index. Obviously indices
supporting primary keys and other unique constraints score really well
here. Indices supporting foreign keys often score poorly.

Each "useful" index is loaded into bitmaps of records. These bitmaps
are ANDed / ORd together to come up with an even more useful bitmap.
Unlike a lot of other DBMS, Firebird can use multiple indices at the
same time for the same bit of the query.

Adam