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eqjoinsel() currently makes use of MCV stats only when we have such stats for both sides of the clause. As coded, though, it would fetch those stats even when they're present for just one side. This can be a bit expensive with high statistics targets, leading to wasted effort in common cases such as joining a unique column to a non-unique column. So it seems worth the trouble to do a quick pre-check to confirm that both sides have MCVs before fetching either. Also, tweak the API spec for get_attstatsslot() to document the method we're using here. David Geier, Tomas Vondra, Tom Lane Discussion: https://postgr.es/m/b9846ca0-5f1c-9b26-5881-aad3f42b07f0@gmail.com
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