排序使用filesort 时查看是否使用了临时表

/* 打开 optimizer_trace，只对本线程有效 */

0:　SET optimizer_trace="enabled=on";

/* @a 保存 Innodb_rows_read 的初始值 */
1： SELECT VARIABLE_VALUE into @a from performance_schema.session_status where variable_name = ‘Innodb_rows_read‘;

/* 执行语句 */
2：SELECT * from titem where fname > 0 ORDER BY fname ;

/* 查看 OPTIMIZER_TRACE 输出  此处有json*/ 
3：SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

/* @b 保存 Innodb_rows_read 的当前值 */

4：select VARIABLE_VALUE into @b from performance_schema.session_status where variable_name = ‘Innodb_rows_read‘;

/* 计算 Innodb_rows_read 差值 */
5：select @b-@a;
SET optimizer_trace="enabled=off";