diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 309389e20d..50205b9bdb 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1378,7 +1378,11 @@ DefineIndex(Oid tableId,
 										 parentIndex->rd_opfamily,
 										 attmap))
 					{
-						Oid			cldConstrOid = InvalidOid;
+						Oid			cldConstrOid;
+
+						cldConstrOid =
+							get_relation_idx_constraint_oid(childRelid,
+															cldidxid);
 
 						/*
 						 * Found a match.
@@ -1387,19 +1391,29 @@ DefineIndex(Oid tableId,
 						 * because of a constraint, then the child needs to
 						 * have a constraint also, so look for one.  If there
 						 * is no such constraint, this index is no good, so
-						 * keep looking.
+						 * keep looking.  On the other hand, if the index that
+						 * exists on the child _is_ a constraint but this one
+						 * isn't, then we shouldn't match them.  XXX should we
+						 * instead choose to ignore the index?
 						 */
 						if (createdConstraintId != InvalidOid)
 						{
-							cldConstrOid =
-								get_relation_idx_constraint_oid(childRelid,
-																cldidxid);
 							if (cldConstrOid == InvalidOid)
 							{
 								index_close(cldidx, lockmode);
 								continue;
 							}
 						}
+						else if (OidIsValid(cldConstrOid))
+							ereport(ERROR,
+									errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+									errmsg("can't create index on partitioned table \"%s\" matching constraint \"%s\" on partition \"%s\"",
+										   RelationGetRelationName(rel),
+										   get_constraint_name(cldConstrOid),
+										   RelationGetRelationName(childrel)),
+									errhint("You can create the index on ONLY \"%s\", or create a constraint instead.",
+											RelationGetRelationName(rel)));
+
 
 						/* Attach index to parent and we're done. */
 						IndexSetParentIndex(cldidx, indexRelationId);
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 66cda26a25..8cb9893e5d 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -18757,6 +18757,10 @@ AttachPartitionEnsureIndexes(List **wqueue, Relation rel, Relation attachrel)
 								 idxRel->rd_opfamily,
 								 attmap))
 			{
+				cldConstrOid =
+					get_relation_idx_constraint_oid(RelationGetRelid(attachrel),
+													cldIdxId);
+
 				/*
 				 * If this index is being created in the parent because of a
 				 * constraint, then the child needs to have a constraint also,
@@ -18765,9 +18769,6 @@ AttachPartitionEnsureIndexes(List **wqueue, Relation rel, Relation attachrel)
 				 */
 				if (OidIsValid(constraintOid))
 				{
-					cldConstrOid =
-						get_relation_idx_constraint_oid(RelationGetRelid(attachrel),
-														cldIdxId);
 					/* no dice */
 					if (!OidIsValid(cldConstrOid))
 						continue;
@@ -18777,6 +18778,22 @@ AttachPartitionEnsureIndexes(List **wqueue, Relation rel, Relation attachrel)
 						get_constraint_type(cldConstrOid))
 						continue;
 				}
+				else
+				{
+					/*
+					 * If the parent does *not* have a constraint, then the
+					 * child must also not have one. Otherwise, things can get
+					 * ugly by detach time.
+					 */
+
+					if (OidIsValid(cldConstrOid))
+						ereport(ERROR,
+								errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+								errmsg("can't attach partition \"%s\" with constraint \"%s\" that matches non-constraint index \"%s\"",
+									   RelationGetRelationName(attachrel),
+									   get_constraint_name(cldConstrOid),
+									   RelationGetRelationName(idxRel)));
+				}
 
 				/* bingo. */
 				IndexSetParentIndex(attachrelIdxRels[i], idx);
@@ -19726,6 +19743,20 @@ ATExecAttachPartitionIdx(List **wqueue, Relation parentIdx, RangeVar *name)
 								   RelationGetRelationName(parentTbl),
 								   RelationGetRelationName(partIdx))));
 		}
+		else
+		{
+			Oid			constroid;
+
+			constroid = get_relation_idx_constraint_oid(RelationGetRelid(partTbl),
+														partIdxId);
+
+			if (OidIsValid(constroid))
+				ereport(ERROR,
+						errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						errmsg("can't attach constraint-supporting index \"%s\" to non-constraint index \"%s\"",
+							   get_constraint_name(constroid),
+							   RelationGetRelationName(partIdx)));
+		}
 
 		/* All good -- do it */
 		IndexSetParentIndex(partIdx, RelationGetRelid(parentIdx));
-- 
2.39.2