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The original coding correctly noted that these aren't just redundancies (they're effectively X IS NOT NULL, assuming = is strict). However, they got treated that way if X happened to be in a single-member EquivalenceClass already, which could happen if there was an ORDER BY X clause, for instance. The simplest and most reliable solution seems to be to not try to process such clauses through the EquivalenceClass machinery; just throw them back for traditional processing. The amount of work that'd be needed to be smarter than that seems out of proportion to the benefit. Per bug #5084 from Bernt Marius Johnsen, and analysis by Andrew Gierth.
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