Andres Freund <> writes:
> Not sure I follow. Unless the values are equivalent (i.e. duplicate key
> values), why should non-uniformity in key space translate to hash space?

Duplicates are exactly the problem.  See estimate_hash_bucket_stats.

> And if there's duplicates it shouldn't hurt much either, unless doing
> a semi/anti-join? All rows are going to be returned and IIRC we quite
> cheaply continue a bucket scan?

If the bucket containing the MCV is bigger than work_mem, you gotta
problem --- one not necessarily shared by the other relation.

                        regards, tom lane

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