Hi Firebird people

I have this select, and I want ask you, why then
the first select don't use the index (id is the primary key)

SELECT SUM(encomendas.qtdtotal) FROM encomendas
WHERE id in (SELECT idenc FROM encgrupolin WHERE id=:xpto)

Plan
PLAN (ENCGRUPOLIN INDEX (RDB$FOREIGN214,FK_ENCGRUPOLIN_ENCOMENDAS))
PLAN (ENCOMENDAS NATURAL)

The in clause don't use indexes?

Thanks

Agostinho
cpis@...