pgsql: Fix sharing Agg transition state of DISTINCT or ordered aggs.

Fix sharing Agg transition state of DISTINCT or ordered aggs.

If a query contained two aggregates that could share the transition value,
we would correctly collect the input into a tuplesort only once, but
incorrectly run the transition function over the accumulated input twice,
in finalize_aggregates(). That caused a crash, when we tried to call
tuplesort_performsort() on an already-freed NULL tuplestore.

Backport to 9.6, where sharing of transition state and this bug were
introduced.

Analysis by Tom Lane.

Discussion: https://www.postgresql.org/message-id/ac5b0b69-744c-9114-6218-8300ac920e61@iki.fi

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ce92fc4e2540d0ce554e498adb27f0ef29199b94

Modified Files
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src/backend/executor/nodeAgg.c | 21 ++++++++++++++++++---
src/test/regress/expected/aggregates.out | 9 +++++++++
src/test/regress/sql/aggregates.sql | 3 +++
3 files changed, 30 insertions(+), 3 deletions(-)