Hello,

d> My problem is when executing the query below, firebird takes much
d> longer time to get a result than MSSQL.

d> SELECT MAX(field_A) as max_A, field_B FROM TABLEX GROUP BY field_B

[...]

d> What should I do to get a better performance with firebird ?

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.



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Best regards,
Pavel Menshchikov
http://www.ls-software.com