executor,table: fix 'show create table' for the temporary table

executor/show.go

@@ -775,7 +775,13 @@ func ConstructResultOfShowCreateTable(ctx sessionctx.Context, tableInfo *model.T
 	}
 
 	sqlMode := ctx.GetSessionVars().SQLMode
-	fmt.Fprintf(buf, "CREATE TABLE %s (\n", stringutil.Escape(tableInfo.Name.O, sqlMode))
+	tableName := stringutil.Escape(tableInfo.Name.O, sqlMode)
+	switch tableInfo.TempTableType {
+	case model.TempTableGlobal:
+		fmt.Fprintf(buf, "CREATE GLOBAL TEMPORARY TABLE %s (\n", tableName)
+	default:
+		fmt.Fprintf(buf, "CREATE TABLE %s (\n", tableName)
+	}
 	var pkCol *model.ColumnInfo
 	var hasAutoIncID bool
 	needAddComma := false
@@ -952,7 +958,14 @@ func ConstructResultOfShowCreateTable(ctx sessionctx.Context, tableInfo *model.T
 
 	buf.WriteString("\n")
 
-	buf.WriteString(") ENGINE=InnoDB")
+	switch tableInfo.TempTableType {
+	case model.TempTableNone:
+		buf.WriteString(") ENGINE=InnoDB")
+	default:
+		// For now the only supported engine for temporary table is memory.
+		buf.WriteString(") ENGINE=memory")
+	}
+
 	// We need to explicitly set the default charset and collation
 	// to make it work on MySQL server which has default collate utf8_general_ci.
 	if len(tblCollate) == 0 || tblCollate == "binary" {
@@ -1009,6 +1022,11 @@ func ConstructResultOfShowCreateTable(ctx sessionctx.Context, tableInfo *model.T
 	if len(tableInfo.Comment) > 0 {
 		fmt.Fprintf(buf, " COMMENT='%s'", format.OutputFormat(tableInfo.Comment))
 	}
+
+	if tableInfo.TempTableType == model.TempTableGlobal {
+		fmt.Fprintf(buf, " ON COMMIT DELETE ROWS")
+	}
+
 	// add partition info here.
 	appendPartitionInfo(tableInfo.Partition, buf)
 	return nil

executor/show_test.go

@@ -1303,3 +1303,25 @@ func (s *testSuite5) TestShowPerformanceSchema(c *C) {
 		testkit.Rows("events_statements_summary_by_digest 0 SCHEMA_NAME 1 SCHEMA_NAME A 0 <nil> <nil> YES BTREE   YES NULL NO",
 			"events_statements_summary_by_digest 0 SCHEMA_NAME 2 DIGEST A 0 <nil> <nil> YES BTREE   YES NULL NO"))
 }
+
+func (s *testSuite5) TestShowTemporaryTable(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("create global temporary table t1 (id int) on commit delete rows")
+	tk.MustExec("create global temporary table t3 (i int primary key, j int) on commit delete rows")
+	// For issue https://github.com/pingcap/tidb/issues/24752
+	tk.MustQuery("show create table t1").Check(testkit.Rows("t1 CREATE GLOBAL TEMPORARY TABLE `t1` (\n" +
+		"  `id` int(11) DEFAULT NULL\n" +
+		") ENGINE=memory DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin ON COMMIT DELETE ROWS"))
+	// No panic, fix issue https://github.com/pingcap/tidb/issues/24788
+	expect := "CREATE GLOBAL TEMPORARY TABLE `t3` (\n" +
+		"  `i` int(11) NOT NULL,\n" +
+		"  `j` int(11) DEFAULT NULL,\n" +
+		"  PRIMARY KEY (`i`) /*T![clustered_index] CLUSTERED */\n" +
+		") ENGINE=memory DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin ON COMMIT DELETE ROWS"
+	tk.MustQuery("show create table t3").Check(testkit.Rows("t3 " + expect))
+
+	// Verify that the `show create table` result can be used to build the table.
+	createTable := strings.ReplaceAll(expect, "t3", "t4")
+	tk.MustExec(createTable)
+}

table/tables/tables.go

@@ -1373,8 +1373,11 @@ func (t *TableCommon) Allocators(ctx sessionctx.Context) autoid.Allocators {
 	} else if ctx.GetSessionVars().IDAllocator == nil {
 		// Use an independent allocator for global temporary tables.
 		if t.meta.TempTableType == model.TempTableGlobal {
-			alloc := ctx.GetSessionVars().GetTemporaryTable(t.meta).GetAutoIDAllocator()
-			return autoid.Allocators{alloc}
+			if alloc := ctx.GetSessionVars().GetTemporaryTable(t.meta).GetAutoIDAllocator(); alloc != nil {
+				return autoid.Allocators{alloc}
+			}
+			// If the session is not in a txn, for example, in "show create table", use the original allocator.
+			// Otherwise the would be a nil pointer dereference.
 		}
 		return t.allocs
 	}