pgsql: Improve eqjoinsel_semi's behavior for small inner relations with

Improve eqjoinsel_semi's behavior for small inner relations with no stats.

If we don't have any MCV statistics for the inner relation, and we don't
trust its numdistinct estimate either, eqjoinsel_semi falls back to a very
conservative estimate (that 50% of the outer rows have matches). This is
particularly problematic if the inner relation is completely empty, since
then even an explicit ANALYZE won't produce any pg_statistic entries,
so there's no way to budge the planner off the bad estimate.

We'd produce a better estimate in such cases if we used the nd2/nd1
selectivity heuristic, so an easy fix is to treat the nd2 estimate as
non-default if we derive it from clamping to the inner rel's rowcount
estimate. This won't fix every related case (mainly because the rowcount
estimate might be larger than DEFAULT_NUM_DISTINCT), but it seems like a
sane extension of the existing logic, so let's apply the change in HEAD
and see if anyone complains. Per bug #14438 from Nikolay Nikitin.

Report: https://postgr.es/m/[email protected]
Discussion: https://postgr.es/m/[email protected]

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ca5f88502096a39f831da74eb02ec9eb3310616d

Modified Files
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src/backend/utils/adt/selfuncs.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)