>Sergey,
>
>I wonder if it is worthwhile to combine the two fields in the expression (as in Craig's example).
>
>>In this case you don't need compound index because you're using each field separately in your WHERE condition.
>>
>>>Is there a benefit in terms of Rushmore in creating compound indices for free tables?
>>>
>>>For example, when we extract invoice headers, part of the SQL statement might contain "where cCustomer = 'ALLAN' and cInvoiceNo = '123456'".
>>>Should there be an index on 'cCustomer + cInvoiceNo' as well as 'cCustomer' and 'cInvoicNo'?
>>>
>>>TIA.

It is worthwhile, if you frequently access for that particular combination.

... where cCustomer = "ALLAN" and cInvoiceNo = '123456'

will do the following: Using the index on the field cCustomer, it will get all index keys that match the expression, cCustomer = "ALLAN". This might be a few hundred or thousand keys.

Next, from the other index, it will get all all invoices that match the condition, cInvoiceNo = "123456". This might also be several index keys.

Finally, it will combine the information in memory, and retrieve the records themselves.

If you use a combined expression, and have the corresponding index, it will retrieve far less index keys, and then get the corresponding records.