MySQL两种临时表的用法详解

外部临时表

通过CREATE TEMPORARY TABLE 创建的临时表，这种临时表称为外部临时表。这种临时表只对当前用户可见，当前会话结束的时候，该临时表会自动关闭。这种临时表的命名与非临时表可以同名（同名后非临时表将对当前会话不可见，直到临时表被删除）。

内部临时表

内部临时表是一种特殊轻量级的临时表，用来进行性能优化。这种临时表会被MySQL自动创建并用来存储某些操作的中间结果。这些操作可能包括在优化阶段或者执行阶段。这种内部表对用户来说是不可见的，但是通过EXPLAIN或者SHOW STATUS可以查看MYSQL是否使用了内部临时表用来帮助完成某个操作。内部临时表在SQL语句的优化过程中扮演着非常重要的角色， MySQL中的很多操作都要依赖于内部临时表来进行优化。但是使用内部临时表需要创建表以及中间数据的存取代价，所以用户在写SQL语句的时候应该尽量的去避免使用临时表。

内部临时表有两种类型：一种是HEAP临时表，这种临时表的所有数据都会存在内存中，对于这种表的操作不需要IO操作。另一种是OnDisk临时表，顾名思义，这种临时表会将数据存储在磁盘上。OnDisk临时表用来处理中间结果比较大的操作。如果HEAP临时表存储的数据大于MAX_HEAP_TABLE_SIZE（详情请参考MySQL手册中系统变量部分），HEAP临时表将会被自动转换成OnDisk临时表。OnDisk临时表在5.7中可以通过INTERNAL_TMP_DISK_STORAGE_ENGINE系统变量选择使用MyISAM引擎或者InnoDB引擎。

本篇文章主要介绍哪些操作可能会利用到内部临时表。如果用户在书写SQL语句的时候能够尽量少的使用内部临时表进行查询优化，将有效的提高查询执行的效率。

首先我们定义一个表t1，

CREATE TABLE t1( a int, b int); INSERT INTO t1 VALUES(1,2),(3,4);

在SQL语句中使用SQL_BUFFER_RESULT hint

SQL_BUFFER_RESULT主要用来让MySQL尽早的释放表上的锁。因为如果数据量很大的话，需要较长时间将数据发送到客户端，通过将数据缓冲到临时表中可以有效的减少读锁对表的占用时间。

例如：

mysql> explain format=json select SQL_BUFFER_RESULT * from t1; EXPLAIN {  "query_block": { "select_id": 1, "cost_info": {  "query_cost": "2.00" }, "buffer_result": {  "using_temporary_table": true,  "table": { "table_name": "t1", "access_type": "ALL", ...

如果SQL语句中包含了DERIVED_TABLE。

在5.7中，由于采用了新的优化方式，我们需要使用 set optimizer_switch='derived_merge=off'来禁止derived table合并到外层的Query中。

例如：

mysql> explain format=json select * from (select * from t1) as tt; EXPLAIN {  "query_block": { "select_id": 1, "cost_info": {  "query_cost": "2.40" }, "table": {  "table_name": "tt",  "access_type": "ALL",  ...  "materialized_from_subquery": { "using_temporary_table": true, ...

我们当前不能使用EXPLAIN来查看是否读取系统表数据需要利用到内部临时表，但是可以通过SHOW STATUS来查看是否利用到了内部临时表。

例如：

mysql> select * from information_schema.character_sets; mysql> show status like 'CREATE%';

mysql> explain format=json select distinct a from t1; EXPLAIN { {  "query_block": { "select_id": 1, "cost_info": {  "query_cost": "1.60" }, "duplicates_removal": {  "using_temporary_table": true, ...

1）如果连接表使用BNL（Batched Nestloop）/BKA(Batched Key Access)

例如：

1))BNL默认是打开的

mysql> explain format=json select * from t1, t1 as t2 order by t1.a; EXPLAIN {  "query_block": {  "select_id": 1,  "cost_info": { "query_cost": "22.00"  },  "ordering_operation": { "using_temporary_table": true,  ...

mysql> set optimizer_switch='block_nested_loop=off'; Query OK, 0 rows affected (0.00 sec) mysql> explain format=json select * from t1, t1 as t2 order by t1.a; EXPLAIN {   "query_block": { "select_id": 1, "cost_info": {  "query_cost": "25.00" }, "ordering_operation": {  "using_filesort": true, ...

例如：

mysql> explain format=json select * from t as t1, t as t2 order by t2.a; EXPLAIN {   "query_block": { "select_id": 1, "cost_info": {  "query_cost": "25.00" }, "ordering_operation": {  "using_temporary_table": true, ...

那么什么样的ORDER BY表达式，MySQL认为是复杂表达式呢？

1))如果排序表达式是SP或者UDF。

例如：

drop function if exists func1; delimiter | create function func1(x int) returns int deterministic begin declare z1, z2 int; set z1 = x; set z2 = z1+2; return z2; end| delimiter ; explain format=json select * from t1 order by func1(a); { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "2.20" }, "ordering_operation": {  "using_temporary_table": true, ...

为了简化执行计划，我们利用INDEX来优化GROUP BY语句。

例如：

 create index idx1 on t1(a);  explain format=json SELECt a FROM t1 group by a order by sum(a);  | {   "query_block": { "select_id": 1, "cost_info": {  "query_cost": "1.20" }, "ordering_operation": {  "using_temporary_table": true,  "using_filesort": true,  "grouping_operation": { "using_filesort": false, ...  drop index idx1 on t1;

例如：

explain format=json select (select rand() from t1 limit 1) as a from t1 order by a; | {  "query_block": { "select_id": 1, "cost_info": {  "query_cost": "1.20" }, "ordering_operation": {  "using_temporary_table": true,  "using_filesort": true, ...

注意: 如果是5.7，我们需要将sql_mode设置为非only_full_group_by模式，否则会报错。

同样为了简化执行计划，我们利用INDEX来优化GROUP BY语句。

例如：

set sql_mode=''; create index idx1 on t1(b); explain format=json select t1.a from t1 group by t1.b order by 1; | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "1.40" }, "ordering_operation": {  "using_temporary_table": true,  "using_filesort": true,  "grouping_operation": { "using_filesort": false, ... drop index idx1 on t1;

1）如果连接表使用BNL（Batched Nestloop）/BKA(Batched Key Access)。

例如：

explain format=json select t2.a from t1, t1 as t2 group by t1.a; | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "8.20" }, "grouping_operation": {  "using_temporary_table": true,  "using_filesort": true,  "cost_info": { "sort_cost": "4.00" ...

例如：

explain format=json select t2.a from t1, t1 as t2 group by t2.a; | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "8.20" }, "grouping_operation": {  "using_temporary_table": true,  "using_filesort": true,  "nested_loop": [ ...

例如：

set sql_mode=''; explain format=json select t1.a from t1 group by t1.b order by t1.a; | {   "query_block": { "select_id": 1, "cost_info": {  "query_cost": "1.40" }, "ordering_operation": {  "using_filesort": true,  "grouping_operation": { "using_temporary_table": true, "using_filesort": false, ...

例如：

explain format=json select sum(t1.a) from t1 left join t1 as t2 on true group by t1.a with rollup; | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "7.20" }, "grouping_operation": {  "using_temporary_table": true,  "using_filesort": true,  "cost_info": { "sort_cost": "4.00"  }, ...

例如：

explain format=json select (select avg(a) from t1) as a from t1 group by a; | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "3.40" }, "grouping_operation": {  "using_temporary_table": true,  "using_filesort": true,  "cost_info": { "sort_cost": "2.00"  }, ...

1) 如果semi-join执行方式为Materialization

例如：

set optimizer_switch='firstmatch=off,duplicateweedout=off'; explain format=json select * from t1 where a in (select b from t1); | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "5.60" }, "nested_loop": [  { "rows_examined_per_scan": 1,  "materialized_from_subquery": { "using_temporary_table": true, "query_block": {  "table": { "table_name": "t1", "access_type": "ALL", ...

例如：

set optimizer_switch='firstmatch=off'; explain format=json select * from t1 where a in (select b from t1); | { "query_block": { "select_id": 1, "cost_info": {  "query_cost": "4.80" }, "duplicates_removal": {  "using_temporary_table": true,  "nested_loop": [ { ...

例如：

explain format=json select * from t1 union select * from t1; | { "query_block": { "union_result": {  "using_temporary_table": true,  "table_name": "", ...

这里Explain不能看到内部临时表被利用，所以需要查看status。

例如：

update t1, t1 as t2 set t1.a=3; show status like 'CREATE%';

1) count(distinct *) 例如： explain format=json select count(distinct a) from t1; 2) group_concat

explain format=json select group_concat(b) from t1;

总结

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