pgsql: Make reduce_outer_joins() smarter about semijoins.

Make reduce_outer_joins() smarter about semijoins.

reduce_outer_joins() mistakenly treated a semijoin like a left join for
purposes of deciding whether not-null constraints created by the join's
quals could be passed down into the join's left-hand side (possibly
resulting in outer-join simplification there).  Actually, semijoin works
like inner join for this purpose, ie, we do not need to see any rows that
can't possibly satisfy the quals.  Hence, two-line fix to treat semi and
inner joins alike.  Per observation by Andres Freund about a performance
gripe from Yazan Suleiman.

Back-patch to 8.4, since this oversight has been there since the current
handling of semijoins was implemented.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=c1da1f45a1033fecd888c56568db5ca9b1c050d2

Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c |   11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)