pgsql: Improve inheritance_planner()'s performance for large inheritanc

Improve inheritance_planner()'s performance for large inheritance sets.

Commit c03ad5602f529787968fa3201b35c119bbc6d782 introduced a planner
performance regression for UPDATE/DELETE on large inheritance sets.
It required copying the append_rel_list (which is of size proportional to
the number of inherited tables) once for each inherited table, thus
resulting in O(N^2) time and memory consumption. While it's difficult to
avoid that in general, the extra work only has to be done for
append_rel_list entries that actually reference subquery RTEs, which
inheritance-set entries will not. So we can buy back essentially all of
the loss in cases without subqueries in FROM; and even for those, the added
work is mainly proportional to the number of UNION ALL subqueries.

Back-patch to 9.2, like the previous commit.

Tom Lane and Dean Rasheed, per a complaint from Thomas Munro.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e538e510e11a6d3aa2a080f36bc46f1cb537532f

Modified Files
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src/backend/optimizer/plan/planner.c | 89 +++++++++++++++++++++++++++++++---
1 file changed, 83 insertions(+), 6 deletions(-)