Hi Svein

Thanks for your anwser
but

SELECT SUM(encomendas.qtdtotal) FROM encomendas
WHERE exists (SELECT * FROM encgrupolin
WHERE encgrupolin.idenc = encomendas.id and
encgrupolin.id=:xpto)

still don't use index on table encomendas

I use Firebird 1.5 RC7

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)

And yes if I replaced my subselect with constants.
eg.
SELECT SUM(encomendas.qtdtotal) FROM encomendas
WHERE encomendas.id in (320,318)

then the plan
is
Plan
PLAN (ENCOMENDAS INDEX (PK_ENCOMENDAS,PK_ENCOMENDAS))

Adapted Plan
PLAN (ENCOMENDAS INDEX (PK_ENCOMENDAS,PK_ENCOMENDAS))

Any sugestion?

Thanks again
Agostinho
cpis@...