pgsql: Fall back to SPI for RI checks with mismatched index collations

Fall back to SPI for RI checks with mismatched index collations

The fast path probes using the referenced index's collation. When that
collation differs from the referenced column's, it can reject valid
references or accept invalid ones. Changing the scan key's collation
is not sufficient, since btree navigation must use the ordering under
which the index was built.

Cache index eligibility lazily after locking the referenced table,
reloading the constraint information, and opening the index. Compare
index and column collations using the held relation descriptors, and
fall back to SPI on a mismatch. Also move the btree eligibility check
here, avoiding a lookup of an index that REINDEX CONCURRENTLY could
drop before the referenced-table lock is acquired.

Cache both acceptance and rejection until the constraint information
is reloaded, so subsequent rows need not repeat the comparisons. Check
before probing a row, and let the caller use SPI if the index is
unsuitable.

Add ICU regression coverage for per-row validation, ordinary inserts,
reordered index columns, and both directions of collation mismatch.

Reported-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA4eK1Lk1DfgiFPL-HccZm-Nm+exg+FcsODH8KPw6BOBuzfHtA@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2bc7b71dc168d537c67a307e5ac5d6c764bbc721

Modified Files
--------------
src/backend/utils/adt/ri_triggers.c | 93 ++++++++++++++++++++++----
src/test/regress/expected/collate.icu.utf8.out | 36 ++++++++++
src/test/regress/sql/collate.icu.utf8.sql | 34 ++++++++++
src/tools/pgindent/typedefs.list | 1 +
4 files changed, 150 insertions(+), 14 deletions(-)