> Yes, SELECT. Can you try yourself ? Construct SELECT statement with WHERE clause that will return e.g. 150 rows,
> and execute it on your 50 000 000 records table, once ORDERed by index, once withou index, and compare the times.
> (and use different fields for search condition and for ordering)
> (for easy switching between ordering with/without index, use ASC/DESC in your order clause)

Ivan, sorting in Firebird has one vulnerability: it operates with
uncompressed records, so if you have field VARCHAR(30000), filled with
average 100 characters, sorting may be inefficient because every time
during merge it will move 30000 bytes. Ordered reading won't.

SY, SD.