Efficient Code

What would be the most efficient way to get records from a large file where
the user can enter selection criteria for as many as 16 fields. i.e.

Key1 is required
Key2 is required
Key3 is required

Field1 is optional
Field2 is optional
Field3 is optional
Field4 is optional
Field5 is optional
Field6 is optional
Field7 is optional
Field8 is optional
Field9 is optional
Field10 is optional
Field11 is optional
Field12 is optional
Field13 is optional

The file currently has over 9 million records and growing.

The file has a record length of almost 300 bytes.

The file currently has 51 logicals but none have the keys I need in the
right sequence.

The screen requires that values be entered for the key values but the
others may  or may not have values entered.

I am not sure if SQL would be the best or making a few logicals over the
file with only the keys and checking the record against the entered data
would be better.

Any opinions?

Thanks in Advance.
Chris Bougher