什么是Internal Temporary？
临时表分为两种，一种是当执行一些SQL的时候MySQL会自动创建的一些中间结果集，称为internal temporary，这些中间结果集可能放在memory中，也有可能放在disk上；
还有一种是手动执行create temporary table 语法生成的外部临时表，这种临时表存储在memory上，数据库shutdown，就会自动删除；
本篇讲的临时表都是指内部临时表，测试使用的MySQL版本是8.0.13；
怎么判断有没有使用内部临时表？
执行计划explain或explain format=json 中出现using temporary;
show status中Created_tmp_disk_tables或Created_tmp_tables数值增加；
什么情况下产生Internal temporary table?
(1)除了后面提到的特殊情况，所有使用union的SQL，但是使用union all没有使用临时表
(2)用到TEMPTABLE算法或者是UNION查询中的视图
mysql> desc select * from t_order union select * from t_group;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| 2 | UNION | t_group | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.01 sec)
但是使用union all没有使用临时表
mysql> desc select * from t_order union all select * from t_group;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | PRIMARY | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| 2 | UNION | t_group | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)(3)使用衍生表
(4)子查询和semi-join
mysql> desc select * from (select * from t_order)t;
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| 2 | DERIVED | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.01 sec)
mysql> desc format=json select * from (select * from t_order)t;
......
"materialized_from_subquery": {
"using_temporary_table": true,(5)order by和group by的子句不一样时，或者表连接中order by或group by的列是被驱动表中的列；
order by和group by 同时使用的时候：
mysql> desc select dept_no from t_order group by dept_no order by dept_no;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary; Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)
或者：
mysql> set session sql_mode='';
Query OK, 0 rows affected (0.00 sec)
mysql> desc select dept_no from t_order group by dept_no order by emp_no;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary; Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)order by 和group by 分别和join使用的时候：
mysql> desc select * from t_group t1 join t_order t2 on t1.emp_no=t2.emp_no order by t2.emp_no;
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+---------------------------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary; Using filesort |
| 1 | SIMPLE | t2 | NULL | ref | ix_t1 | ix_t1 | 5 | employees.t1.emp_no | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+---------------------------------+
2 rows in set, 1 warning (0.00 sec)
mysql> desc select * from t_group t1 join t_order t2 on t1.emp_no=t2.emp_no order by t1.emp_no;
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+----------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using filesort |
| 1 | SIMPLE | t2 | NULL | ref | ix_t1 | ix_t1 | 5 | employees.t1.emp_no | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+----------------+
2 rows in set, 1 warning (0.00 sec)
mysql> desc select t1.dept_no from t_group t1 join t_order t2 on t1.emp_no=t2.emp_no group by t1.dept_no;
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary |
| 1 | SIMPLE | t2 | NULL | ref | ix_t1 | ix_t1 | 5 | employees.t1.emp_no | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
2 rows in set, 1 warning (0.00 sec)
mysql> desc select t2.dept_no from t_group t1 join t_order t2 on t1.emp_no=t2.emp_no group by t2.dept_no;
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary |
| 1 | SIMPLE | t2 | NULL | ref | ix_t1 | ix_t1 | 5 | employees.t1.emp_no | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------------+------+----------+-----------------+
2 rows in set, 1 warning (0.00 sec)(6)使用distinct或者distinct集合ORDER BY时
mysql> desc select distinct * from t_order;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)(7)SQL中用到SQL_SMALL_RESULT选项时；
(8)INSERT ... SELECT针对同一个表操作的时候
mysql> desc insert into t_order select * from t_order;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | INSERT | t_order | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 1 | SIMPLE | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using temporary |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
2 rows in set, 1 warning (0.00 sec)(9)使用GROUP_CONCAT() or COUNT(DISTINCT)
使用group_concat()时产生临时表：
mysql> flush status;
Query OK, 0 rows affected (0.02 sec)
mysql> select dept_no,group_concat(emp_no) from t_order group by dept_no;
+---------+-------------------------+
| dept_no | group_concat(emp_no) |
+---------+-------------------------+
| d002 | 31112 |
| d004 | 10004 |
| d005 | 24007,30970,40983,50449 |
| d006 | 22744 |
| d007 | 49667 |
| d008 | 48317 |
+---------+-------------------------+
6 rows in set (0.00 sec)
mysql> show status like '%tmp%';
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0 |
| Created_tmp_files | 0 |
| Created_tmp_tables | 1 |
+-------------------------+-------+
3 rows in set (0.00 sec)使用count(distinct)时产生临时表：
mysql> flush status;
Query OK, 0 rows affected (0.02 sec)
mysql> desc select count(distinct dept_no) from t_order;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | t_order | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> show status like '%tmp%';
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0 |
| Created_tmp_files | 0 |
| Created_tmp_tables | 1 |
+-------------------------+-------+
3 rows in set (0.01 sec)什么情况下产生的内部临时表不是在内存中，而是在磁盘上？
(1)表存在blob或text字段；
(2)在SELECT UNION、UNION ALL查询中，存在最大长度超过512的列（对于字符串类型是512个字符，对于二进制类型则是512字节）；
(3)使用show columns和describe命令在存在blob列的表上；
内部临时表使用什么存储引擎？
MySQL8.0.2开始支持internal_tmp_mem_storage_engine参数；
(1)当internal_tmp_mem_storage_engine=TempTable时，
TempTable存储引擎为varchar和varbinary数据类型提供高效的存储，temptable_max_ram=1G定义临时表最大可以使用的内存空间，但是如果参数temptable_use_mma=on，则表示可以继续使用内存存储临时表，如果off，则临时表超过阈值，只能使用磁盘存储；
(2)当internal_tmp_mem_storage_engine=memory时：
内部临时表大小超过参数tmp_table_size和max_heap_table_size时候，会自动从内存中转移到磁盘上，内部临时表在磁盘上默认使用的是innodb存储引擎，由参数internal_tmp_disk_storage_engine决定.
参考链接
Internal Temporary Table Use in MySQL
