How does Filemaker query external tables?

Situation:

in FMP Table A = 2100 records

External

Table B = 900000 records

Table C = 9Mill records

Table A calculated field B = if Table A field A is empty then field B value = value from Table C field b else table A field b = "No Value" (I'm doing this is a case statement. The actual calculation is a little more complex than I have stated.)

The relationship is table A keyfield1 = Table B keyfield1. Table B keyfield2 = table C keyfield2.

I have a layout that displays the calculated field. My question is when I sort this layout by calculated Field b it takes forever. The DBA here at worked wanted to know how FMP queries the oracle tables. Does FMP do a separate query for each row in Table A that appears in my layout or does it do something else?

We are having oracle issues here at work and I want to make sure that I'm minimizing my impact on the system.