>>I'm not sure, I understand you. UNION uses all fields to compare records to eliminate duplicates. UNION ALL doesn't compare them at all. It's just joins separate queries results into one record set (cursor).
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>If it is bound on all fields, then, I don't see the need for it. I would have assumes it would have gone by the first field, as in general, it's the primary key, and that would have done it.

Not necessarily so. The source for the select may be anything - a result from a previous SQL, a XmlToCursor() cursor, a SPT cursor - and there's no way Fox could rely on any key there to be a key of any sort, specially when none of them (nor the regular tables) are required to have indexes of any kind. And it does have the requirement to work in all of those cases - so it has to compare all fields. It has done so since 2.0.