@Joachim: did you see that it is the difference between 2 fields? PIEZAS_D-SALDO is an expression and I don't think an index can help here a lot.
Maybe you should experiment with equivilent expressions:
"PIEZAS_D-SALDO>0" is identical to "PIEZAS_D > SALDO", maybe that helps?
If you would store the difference this could definitely be indexed....