pgsql: Restrict RI fast-path FK check to btree referenced indexes

Restrict RI fast-path FK check to btree referenced indexes

The RI fast-path check probes the referenced index directly and, for
single-column keys, uses SK_SEARCHARRAY. Both assume the index is a
btree. A comment claimed "PK indexes are always btree", but a foreign
key's referenced index need not be a primary key:
transformFkeyCheckAttrs() accepts any unique (or, for temporal keys,
exclusion) index, so an out-of-tree access method advertising
amcanunique could supply a non-btree index reachable by the fast path.

Add pk_index_is_btree to RI_ConstraintInfo, set from the referenced
index's access method when the constraint is loaded, and make
ri_fastpath_is_applicable() return false for non-btree indexes so such
constraints fall back to the SPI path.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260705210533.ee.noahmisch@microsoft.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8c0aa08c159494a86fe5c3206e8a9f5de850112e

Modified Files
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src/backend/utils/adt/ri_triggers.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)