pgsql: Be more wary of missing statistics in eqjoinsel_semi().

Be more wary of missing statistics in eqjoinsel_semi().

In particular, if we don't have real ndistinct estimates for both sides,
fall back to assuming that half of the left-hand rows have join partners.
This is what was done in 8.2 and 8.3 (cf nulltestsel() in those versions).
It's pretty stupid but it won't lead us to think that an antijoin produces
no rows out, as seen in recent example from Uwe Schroeder.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/159c47dc7170110a39f8a16b1d0b7811f5556f87

Modified Files
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src/backend/utils/adt/selfuncs.c | 49 ++++++++++++++++++++++++-------------
1 files changed, 32 insertions(+), 17 deletions(-)