Thanks for your reply.

> Try to add DESC index on field_A: in FB indices are uni-directional.
> If field_A has a poor selectivity, add compound DESC index on
> (field_A, primary_key) instead of adding DESC index on field_A.

I think What is strainge is that,
They are same databases in MSSQL and firebird,
with same tables, same indices, same data.

but MSSQL uses memory as 10 times as Firebird uses.