Test, don't just Assert, that mergejoin's inputs are in order.

tglsfdc · tglsfdc · commit 5e26aa641e8a · 2022-02-05T11:59:29.000-05:00
There are two Asserts in nodeMergejoin.c that are reachable if the input data is not in the expected order. This seems way too fragile. Alexander Lakhin reported a case where the assertions could be triggered with misconfigured foreign-table partitions, and bitter experience with unstable operating system collation definitions suggests another easy route to hitting them. Neither Assert is in a place where we can't afford one more test-and-branch, so replace 'em with plain test-and-elog logic. Per bug #17395. While the reported symptom is relatively recent, collation changes could happen anytime, so back-patch to all supported branches. Discussion: https://postgr.es/m/17395-8c326292078d1a57@postgresql.org
diff --git a/src/backend/executor/nodeMergejoin.c b/src/backend/executor/nodeMergejoin.c
@@ -893,11 +893,10 @@ ExecMergeJoin(PlanState *pstate)
 
 						if (compareResult == 0)
 							node->mj_JoinState = EXEC_MJ_JOINTUPLES;
-						else
-						{
-							Assert(compareResult < 0);
+						else if (compareResult < 0)
 							node->mj_JoinState = EXEC_MJ_NEXTOUTER;
-						}
+						else	/* compareResult > 0 should not happen */
+							elog(ERROR, "mergejoin input data is out of order");
 						break;
 					case MJEVAL_NONMATCHABLE:
 
@@ -1087,7 +1086,7 @@ ExecMergeJoin(PlanState *pstate)
 
 					node->mj_JoinState = EXEC_MJ_JOINTUPLES;
 				}
-				else
+				else if (compareResult > 0)
 				{
 					/* ----------------
 					 *	if the new outer tuple didn't match the marked inner
@@ -1106,7 +1105,6 @@ ExecMergeJoin(PlanState *pstate)
 					 *	no more inners, no more matches are possible.
 					 * ----------------
 					 */
-					Assert(compareResult > 0);
 					innerTupleSlot = node->mj_InnerTupleSlot;
 
 					/* reload comparison data for current inner */
@@ -1140,6 +1138,8 @@ ExecMergeJoin(PlanState *pstate)
 							return NULL;
 					}
 				}
+				else			/* compareResult < 0 should not happen */
+					elog(ERROR, "mergejoin input data is out of order");
 				break;
 
 				/*----------------------------------------------------------
