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Clamp semijoin selectivity to be not more than inner-join selectivity.

We should never estimate the output of a semijoin to be more rows than we estimate for an inner join with the same input rels and join condition; it's obviously impossible for that to happen. However, given the relatively poor quality of our semijoin selectivity estimates --- particularly, but not only, in cases where we punt and return a default estimate --- we did often deliver such estimates. To improve matters, calculate both estimates inside eqjoinsel() and take the smaller one. The bulk of this patch is just mechanical refactoring to avoid repetitive information lookup when we call both eqjoinsel_semi and eqjoinsel_inner. The actual new behavior is just selec = Min(selec, inner_rel->rows * selec_inner); which looks a bit odd but is correct because of our different definitions for inner and semi join selectivity. There is one ensuing plan change in the regression tests, but it looks reasonable enough (and checking the actual row counts shows that the estimate moved closer to reality, not further away). Per bug #15160 from Alexey Ermakov. Although this is arguably a bug fix, I won't risk destabilizing plan choices in stable branches by back-patching. Tom Lane, reviewed by Melanie Plageman Discussion: https://postgr.es/m/152395805004.19366.3107109716821067806@wrigleys.postgresql.org

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src
- backend/utils/adt
  - selfuncs.c
- test/regress/expected
  - partition_join.out

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