pgsql: Preserve typmod and collation when reducing an expression to its

Preserve typmod and collation when reducing an expression to its input

eval_const_expressions() can reduce a COALESCE, CASE, or NULLIF
expression to one of its inputs. The input, however, does not
necessarily expose the same typmod and collation as the expression it
replaces. In particular, an explicit COLLATE clause on one input
determines the collation of the whole expression, while the other
inputs keep their own collations. If the expression was reduced to
one of those other inputs, anything that relies on exprCollation(),
such as sorting or grouping on the expression, would use the wrong
collation and could produce wrong results.

For CASE and NULLIF this is a long-standing oversight. For COALESCE
it has become much easier to hit since commits 931766aae and
10c4fe074, which reduce a COALESCE with a single surviving argument,
or with a leading non-nullable argument, to that argument.

To fix, apply applyRelabelType() to the replacement in these cases, so
that it exposes the type, typmod, and collation of the expression it
replaces.

Back-patch to v19 only. The CASE and NULLIF cases exist in older
branches too, but they are not back-patched further, given the lack of
field reports and the fact that the fix visibly changes the typmod
exposed by a reduced CASE expression.

Reported-by: Shihao Zhong <zhong950419(at)gmail(dot)com>
Author: Shihao Zhong <zhong950419(at)gmail(dot)com>
Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqR7VXacmRP41UBaM5u4aseeCH5ECVmQgn_YS=L7=xdhTw@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7d0d257fd9f6c0d8282873715168a8366f718150

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 48 ++++++++++++++++++--------
src/test/regress/expected/collate.icu.utf8.out | 25 ++++++++++++++
src/test/regress/sql/collate.icu.utf8.sql | 13 +++++++
3 files changed, 71 insertions(+), 15 deletions(-)