>>EMPTY is optimizable if there is an index with INDEX ON EMPTY(FIELD).
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>Right, of course, but who would create such index? That's why I didn't even mention it.
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>EMPTY is noted as not optimizable in the help as a note near BETWEEN and INLIST because EMPTY does not optimize the way BETWEEN and INLIST do. That is, those functions will use an INDEX ON FIELD.
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>Yes. Or you can use IN (....), which now can hold unlimited (limited by memory) number of items.

People are saying EMPTY is not optimizable as if it is an undeniable fact. As to who might create such an index, how about those same people who use EMPTY() in a filter/where despite the "fact" that it cannot be optimized?