pgsql: Fix improper repetition of previous results from a hashed aggreg

Fix improper repetition of previous results from a hashed aggregate.

ExecReScanAgg's check for whether it could re-use a previously calculated
hashtable neglected the possibility that the Agg node might reference
PARAM_EXEC Params that are not referenced by its input plan node.  That's
okay if the Params are in upper tlist or qual expressions; but if one
appears in aggregate input expressions, then the hashtable contents need
to be recomputed when the Param's value changes.

To avoid unnecessary performance degradation in the case of a Param that
isn't within an aggregate input, add logic to the planner to determine
which Params are within aggregate inputs.  This requires a new field in
struct Agg, but fortunately we never write plans to disk, so this isn't
an initdb-forcing change.

Per report from Jeevan Chalke.  This has been broken since forever,
so back-patch to all supported branches.

Andrew Gierth, with minor adjustments by me

Report: <CAM2+6=VY8ykfLT5Q8vb9B6EbeBk-NGuLbT6seaQ+Fq4zXvrDcA@mail.gmail.com>

Branch
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REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/616be05dfea09e8221e190086c0d75351f3a57ca

Modified Files
--------------
src/backend/executor/nodeAgg.c           | 10 +++--
src/backend/nodes/copyfuncs.c            |  1 +
src/backend/nodes/outfuncs.c             |  1 +
src/backend/nodes/readfuncs.c            |  1 +
src/backend/optimizer/plan/createplan.c  |  1 +
src/backend/optimizer/plan/subselect.c   | 48 +++++++++++++++++++-
src/include/nodes/plannodes.h            |  3 +-
src/test/regress/expected/aggregates.out | 75 ++++++++++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql      | 22 ++++++++++
9 files changed, 156 insertions(+), 6 deletions(-)