relate4query

I am using a DBF that has the following structure:

field1 Numeric 5 0
field2 Char 5

The DBF has an index CDX. The key expression is str(field1,6)+field2
I use function relate4queryset("str(field1,5) <> ' 0'").
Why function relate4optimizeable return 0 (Not optimizeable) ?

Thank you for the help

Regards
Catello
Italia

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