Allow non-btree unique indexes for matviews

We were rejecting non-btree indexes in some cases owing to the
inability to determine the equality operators for other index AMs;
that problem no longer exists, because we can look up the equality
operator using COMPARE_EQ.

Stop rejecting these indexes, but instead rely on all unique indexes
having equality operators.  Unique indexes must have equality
operators.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com

src/backend/commands/matview.c

@@ -774,16 +774,14 @@ refresh_by_match_merge(Oid matviewOid, Oid tempOid, Oid relowner,
 				if (!HeapTupleIsValid(cla_ht))
 					elog(ERROR, "cache lookup failed for opclass %u", opclass);
 				cla_tup = (Form_pg_opclass) GETSTRUCT(cla_ht);
-				Assert(cla_tup->opcmethod == BTREE_AM_OID);
 				opfamily = cla_tup->opcfamily;
 				opcintype = cla_tup->opcintype;
 				ReleaseSysCache(cla_ht);
 
-				op = get_opfamily_member(opfamily, opcintype, opcintype,
-										 BTEqualStrategyNumber);
+				op = get_opfamily_member_for_cmptype(opfamily, opcintype, opcintype, COMPARE_EQ);
 				if (!OidIsValid(op))
-					elog(ERROR, "missing operator %d(%u,%u) in opfamily %u",
-						 BTEqualStrategyNumber, opcintype, opcintype, opfamily);
+					elog(ERROR, "missing equality operator for (%u,%u) in opfamily %u",
+						 opcintype, opcintype, opfamily);
 
 				/*
 				 * If we find the same column with the same equality semantics
@@ -920,15 +918,10 @@ is_usable_unique_index(Relation indexRel)
 
 	/*
 	 * Must be unique, valid, immediate, non-partial, and be defined over
-	 * plain user columns (not expressions).  We also require it to be a
-	 * btree.  Even if we had any other unique index kinds, we'd not know how
-	 * to identify the corresponding equality operator, nor could we be sure
-	 * that the planner could implement the required FULL JOIN with non-btree
-	 * operators.
+	 * plain user columns (not expressions).
 	 */
 	if (indexStruct->indisunique &&
 		indexStruct->indimmediate &&
-		indexRel->rd_rel->relam == BTREE_AM_OID &&
 		indexStruct->indisvalid &&
 		RelationGetIndexPredicate(indexRel) == NIL &&
 		indexStruct->indnatts > 0)