Thanks for all the inputs, guys !!

The real case is that the table has supplier invoices, "id_proveedor" is the
supplier's ID and "saldo" is the invoice balance (im spanish speaking). So the
table will grow with the time, but always the invoices with balance <> 0 will be
just a few. So answering to Svein question: at the begining it will make no
sence to avoid the invoices with saldo = 0, but in the future, as the table
grows, I think it would be faster if I only iterate trough the invoices with a
balance different to zero of certain supplier They will be, in most of the cases
just a few, while the total invoices may be a very big number with the years...

So your advice is that I should live FB alone a let it decide what index to use,
or should I try to find the way of creating a usefull index for my prupose?

Thanks again!!

-s