IF on a referenced field

[Simon_Bailey]

I have a referenced field on a table with 24,000 rows.

[Postcode Area REF].[CC]

I know only about 6,000 rows will return a value, which can be checked with a value in the table

If I use an IF before the reference will that make any difference to the speed?

IF([UK Sales Area]=SOUTH,[Postcode Area REF].[CC],"")

I assume it should be quicker as it doesn't have to check the other table for every row, but it does have another operation(?) to do on every row.

[1minManager]

Probably not, as it will still check all rows.  It would be better to create a slice then use that in the Ref

[Simon_Bailey]

I can't use a slice. It's the table the row is in that is large. the table it's referencing is only small.