>>What if you change the IN for INLIST()?
>>>Actually, it was my mistake. Somehow I lost NOT before IN...
>
>I think both should be equally optimized, right? IN and INLIST are intechangable...
>
>I'm using VFP8 SP1 for this application...

Not sure about IN, but I am sure about INLIST for it is said so in the help (I could not find the same for IN, but did not look too much)

A basic optimizable expression has the following characteristics: 

eIndex exactly matches the expression on which an index is constructed.

eExpr is any expression and can include variables and fields from other unrelated tables.

relOp is one of the following relational operators: <, >, =, <=, >=, <>, #, ==, or !=. You can also use the ISNULL( ), BETWEEN( ), or INLIST( ) functions (or their SQL equivalents such as IS NULL, and so on).

You can use BETWEEN( ) or INLIST( ) in the following two forms:

  Copy Code 
BETWEEN(eIndex, eExpr, eExpr)
 

-or-

  Copy Code 
INLIST(eIndex, eExpr [, eExpr, eExpr, ...])
 

Although it says there (or their SQL equivalents such as IS NULL, and so on) so you might be very well right