Restore the previous semantics of get_constraint_index().

Commit 8b069ef5d changed this function to look at pg_constraint.conindid
rather than searching pg_depend.  That was a good performance improvement,
but it failed to preserve the exact semantics.  The old code would only
return an index that was "owned by" (internally dependent on) the
specified constraint, whereas the new code will also return indexes that
are just referenced by foreign key constraints.  This confuses ALTER
TABLE, which was implicitly expecting the previous semantics, into
failing with errors like
    ERROR:  relation 146621 has multiple clustered indexes
or
    ERROR:  "pk_attbl" is not an index for table "atref"

We can fix this without reverting the performance improvement by adding
a contype check in get_constraint_index().  Another way could be to
make ALTER TABLE check it, but I'm worried that extension code could
also have subtle dependencies on the old semantics.

Tom Lane and Japin Li, per bug #17409 from Holly Roberts.
Back-patch to v14 where the error crept in.

Discussion: https://postgr.es/m/17409-52871dda8b5741cb@postgresql.org

1 files changed, 13 insertions, 3 deletions

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 6bba5f8ec4e..5564b53f257 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -1126,8 +1126,13 @@ get_constraint_name(Oid conoid)
  *		Given the OID of a unique, primary-key, or exclusion constraint,
  *		return the OID of the underlying index.
  *
- * Return InvalidOid if the index couldn't be found; this suggests the
- * given OID is bogus, but we leave it to caller to decide what to do.
+ * Returns InvalidOid if the constraint could not be found or is of
+ * the wrong type.
+ *
+ * The intent of this function is to return the index "owned" by the
+ * specified constraint.  Therefore we must check contype, since some
+ * pg_constraint entries (e.g. for foreign-key constraints) store the
+ * OID of an index that is referenced but not owned by the constraint.
  */
 Oid
 get_constraint_index(Oid conoid)
@@ -1140,7 +1145,12 @@ get_constraint_index(Oid conoid)
 		Form_pg_constraint contup = (Form_pg_constraint) GETSTRUCT(tp);
 		Oid			result;
 
-		result = contup->conindid;
+		if (contup->contype == CONSTRAINT_UNIQUE ||
+			contup->contype == CONSTRAINT_PRIMARY ||
+			contup->contype == CONSTRAINT_EXCLUSION)
+			result = contup->conindid;
+		else
+			result = InvalidOid;
 		ReleaseSysCache(tp);
 		return result;
 	}