pgsql: Postpone aggregate checks until after collation is assigned.

Postpone aggregate checks until after collation is assigned.

Previously, parseCheckAggregates was run before
assign_query_collations, but this causes problems if any expression
has already had a collation assigned by some transform function (e.g.
transformCaseExpr) before parseCheckAggregates runs. The differing
collations would cause expressions not to be recognized as equal to
the ones in the GROUP BY clause, leading to spurious errors about
unaggregated column references.

The result was that CASE expr WHEN val ... would fail when "expr"
contained a GROUPING() expression or matched one of the group by
expressions, and where collatable types were involved; whereas the
supposedly identical CASE WHEN expr = val ... would succeed.

Backpatch all the way; this appears to have been wrong ever since
collations were introduced.

Per report from Guillaume Lelarge, analysis and patch by me.

Discussion: https://postgr.es/m/CAECtzeVSO_US8C2Khgfv54ZMUOBR4sWq+6_bLrETnWExHT=rFg@mail.gmail.com
Discussion: https://postgr.es/m/87muo0k0c7.fsf@news-spur.riddles.org.uk

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/409230a721cf32a2df4fd02338e785b3993f9c7a

Modified Files
--------------
src/backend/parser/analyze.c               | 18 ++++++++++++------
src/test/regress/expected/aggregates.out   | 19 +++++++++++++++++++
src/test/regress/expected/groupingsets.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql        |  8 ++++++++
src/test/regress/sql/groupingsets.sql      | 11 +++++++++++
5 files changed, 75 insertions(+), 6 deletions(-)