diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 241cc8e68e..7ab3a51136 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -355,6 +355,7 @@ static CoercionPathType findFkeyCast(Oid targetTypeId, Oid sourceTypeId,
 static void validateForeignKeyConstraint(char *conname,
 										 Relation rel, Relation pkrel,
 										 Oid pkindOid, Oid constraintOid);
+static void CheckAlterTableIsSafe(Relation rel);
 static void ATController(AlterTableStmt *parsetree,
 						 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode,
 						 AlterTableUtilityContext *context);
@@ -3697,6 +3698,37 @@ CheckTableNotInUse(Relation rel, const char *stmt)
 						stmt, RelationGetRelationName(rel))));
 }
 
+/*
+ * CheckAlterTableIsSafe
+ *		Verify that it's safe to allow ALTER TABLE on this relation.
+ *
+ * This consists of CheckTableNotInUse() plus a check that the relation
+ * isn't another session's temp table.  We must split out the temp-table
+ * check because there are callers of CheckTableNotInUse() that don't want
+ * that, notably DROP TABLE.  (We must allow DROP or we couldn't clean out
+ * an orphaned temp schema.)  Compare truncate_check_activity().
+ */
+static void
+CheckAlterTableIsSafe(Relation rel)
+{
+	/*
+	 * Don't allow ALTER on temp tables of other backends.  Their local buffer
+	 * manager is not going to cope if we need to change the table's contents.
+	 * Even if we don't, there may be optimizations that assume temp tables
+	 * aren't subject to such interference.
+	 */
+	if (RELATION_IS_OTHER_TEMP(rel))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot alter temporary tables of other sessions")));
+
+	/*
+	 * Also check for active uses of the relation in the current transaction,
+	 * including open scans and pending AFTER trigger events.
+	 */
+	CheckTableNotInUse(rel, "ALTER TABLE");
+}
+
 /*
  * AlterTableLookupRelation
  *		Look up, and lock, the OID for the relation named by an alter table
@@ -3771,7 +3803,7 @@ AlterTable(AlterTableStmt *stmt, LOCKMODE lockmode,
 	/* Caller is required to provide an adequate lock. */
 	rel = relation_open(context->relid, NoLock);
 
-	CheckTableNotInUse(rel, "ALTER TABLE");
+	CheckAlterTableIsSafe(rel);
 
 	ATController(stmt, rel, stmt->cmds, stmt->relation->inh, lockmode, context);
 }
@@ -5092,7 +5124,9 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 
 			/*
 			 * Don't allow rewrite on temp tables of other backends ... their
-			 * local buffer manager is not going to cope.
+			 * local buffer manager is not going to cope.  (This is redundant
+			 * with the check in CheckAlterTableIsSafe, but for safety we'll
+			 * check here too.)
 			 */
 			if (RELATION_IS_OTHER_TEMP(OldHeap))
 				ereport(ERROR,
@@ -5835,7 +5869,7 @@ ATSimpleRecursion(List **wqueue, Relation rel,
 				continue;
 			/* find_all_inheritors already got lock */
 			childrel = relation_open(childrelid, NoLock);
-			CheckTableNotInUse(childrel, "ALTER TABLE");
+			CheckAlterTableIsSafe(childrel);
 			ATPrepCmd(wqueue, childrel, cmd, false, true, lockmode, context);
 			relation_close(childrel, NoLock);
 		}
@@ -5844,7 +5878,7 @@ ATSimpleRecursion(List **wqueue, Relation rel,
 
 /*
  * Obtain list of partitions of the given table, locking them all at the given
- * lockmode and ensuring that they all pass CheckTableNotInUse.
+ * lockmode and ensuring that they all pass CheckAlterTableIsSafe.
  *
  * This function is a no-op if the given relation is not a partitioned table;
  * in particular, nothing is done if it's a legacy inheritance parent.
@@ -5865,7 +5899,7 @@ ATCheckPartitionsNotInUse(Relation rel, LOCKMODE lockmode)
 
 			/* find_all_inheritors already got lock */
 			childrel = table_open(lfirst_oid(cell), NoLock);
-			CheckTableNotInUse(childrel, "ALTER TABLE");
+			CheckAlterTableIsSafe(childrel);
 			table_close(childrel, NoLock);
 		}
 		list_free(inh);
@@ -5898,7 +5932,7 @@ ATTypedTableRecursion(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		Relation	childrel;
 
 		childrel = relation_open(childrelid, lockmode);
-		CheckTableNotInUse(childrel, "ALTER TABLE");
+		CheckAlterTableIsSafe(childrel);
 		ATPrepCmd(wqueue, childrel, cmd, true, true, lockmode, context);
 		relation_close(childrel, NoLock);
 	}
@@ -6591,7 +6625,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* find_inheritance_children already got lock */
 		childrel = table_open(childrelid, NoLock);
-		CheckTableNotInUse(childrel, "ALTER TABLE");
+		CheckAlterTableIsSafe(childrel);
 
 		/* Find or create work queue entry for this table */
 		childtab = ATGetQueueEntry(wqueue, childrel);
@@ -8050,7 +8084,7 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 
 			/* find_inheritance_children already got lock */
 			childrel = table_open(childrelid, NoLock);
-			CheckTableNotInUse(childrel, "ALTER TABLE");
+			CheckAlterTableIsSafe(childrel);
 
 			tuple = SearchSysCacheCopyAttName(childrelid, colName);
 			if (!HeapTupleIsValid(tuple))	/* shouldn't happen */
@@ -8508,7 +8542,7 @@ ATAddCheckConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
 
 		/* find_inheritance_children already got lock */
 		childrel = table_open(childrelid, NoLock);
-		CheckTableNotInUse(childrel, "ALTER TABLE");
+		CheckAlterTableIsSafe(childrel);
 
 		/* Find or create work queue entry for this table */
 		childtab = ATGetQueueEntry(wqueue, childrel);
@@ -9258,7 +9292,7 @@ addFkRecurseReferencing(List **wqueue, Constraint *fkconstraint, Relation rel,
 						referenced;
 			ListCell   *cell;
 
-			CheckTableNotInUse(partition, "ALTER TABLE");
+			CheckAlterTableIsSafe(partition);
 
 			attmap = build_attrmap_by_name(RelationGetDescr(partition),
 										   RelationGetDescr(rel));
@@ -11131,7 +11165,7 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 
 			/* Must match lock taken by RemoveTriggerById: */
 			frel = table_open(con->confrelid, AccessExclusiveLock);
-			CheckTableNotInUse(frel, "ALTER TABLE");
+			CheckAlterTableIsSafe(frel);
 			table_close(frel, NoLock);
 		}
 
@@ -11209,7 +11243,7 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 
 		/* find_inheritance_children already got lock */
 		childrel = table_open(childrelid, NoLock);
-		CheckTableNotInUse(childrel, "ALTER TABLE");
+		CheckAlterTableIsSafe(childrel);
 
 		ScanKeyInit(&skey[0],
 					Anum_pg_constraint_conrelid,
@@ -11512,7 +11546,7 @@ ATPrepAlterColumnType(List **wqueue,
 
 			/* find_all_inheritors already got lock */
 			childrel = relation_open(childrelid, NoLock);
-			CheckTableNotInUse(childrel, "ALTER TABLE");
+			CheckAlterTableIsSafe(childrel);
 
 			/*
 			 * Verify that the child doesn't have any inherited definitions of