**摘要：**在数据库查询中,往往会需要查询多个表的数据,比如查询会员信息同时查询关于这个会员的订单信息,如果分语句查询的话,效率会很低,就需要用到join关键字来连表查询了。
Join并行
Join并行1. 多表join介绍2. 多表Join的方式不使用Join buffer使用Join buffer3. Join执行流程（老执行器）
1. 多表join介绍
JOIN子句用于根据两个或多个表之间的相关列来组合它们。 例如：
Orders：
Customers：
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;
2. 多表Join的方式
Hash join使用新执行器实现，在这里不做讨论
MySQL支持的都是Nested-Loop Join，以及它的变种。
不使用Join buffer
a) Simple Nested-Loop
对r表的每一行，完整扫描s表，根据r[i]-s[i]组成的行去判断是否满足条件，并返回满足条件的结果给客户端。
mysql> show create table t1;
+-------+----------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+----------------------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+----------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table t3;
+-------+--------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------+
| t3 | CREATE TABLE `t3` (
`id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+--------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> explain select /*+ NO_BNL() */ * from t1, t3 where t1.id = t3.id;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100.00 | NULL |
| 1 | SIMPLE | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
b) Index Nested-Loop
对r表的每一行，先根据连接条件去查询s表索引，然后回表查到匹配的数据，并返回满足条件的结果给客户端。
mysql> show create table t2;
+-------+---------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+---------------------------------------------------------------------------------------------------------------------------------------+
| t2 | CREATE TABLE `t2` (
`id` int(11) NOT NULL,
KEY `index1` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+---------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> explain select * from t1, t2 where t1.id = t2.id;
+----+-------------+-------+------------+------+---------------+--------+---------+------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+--------+---------+------------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | ref | index1 | index1 | 4 | test.t1.id | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+--------+---------+------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
使用Join buffer
a) Block Nested Loop
从r表读取一部分数据到join cache中，当r表数据读完或者join cache满后，做join操作。
b) Batched Key Access
从r表读取一部分数据到join cache中，s表中记录r表被连接的列的值作为索引，查询所有符合条件的索引，然后将这些符合条件的索引排序，然后统一回表查询记录。
其中，对于每一个cached record，都会有一个key，通过这个key去s表扫描所需的数据。
dsmrr_fill_buffer(){
while((rowids_buf_cur < rowids_buf_end) &&
!(res = h2->handler::multi_range_read_next(&range_info))){
//下压的index条件
if (h2->mrr_funcs.skip_index_tuple &&
h2->mrr_funcs.skip_index_tuple(h2->mrr_iter, curr_range->ptr))
continue;
memcpy(rowids_buf_cur, h2->ref, h2->ref_length);
}
varlen_sort(
rowids_buf, rowids_buf_cur, elem_size,
[this](const uchar *a, const uchar *b) { return h->cmp_ref(a, b) < 0; });
}
dsmrr_next(){
do{
if (rowids_buf_cur == rowids_buf_last) {
dsmrr_fill_buffer();
}
// first match
if (h2->mrr_funcs.skip_record &&
h2->mrr_funcs.skip_record(h2->mrr_iter, (char *)cur_range_info, rowid))
continue;
res = h->ha_rnd_pos(table->record[0], rowid);
break;
} while(true);
}
JOIN_CACHE_BKA::join_matching_records(){
while (!(error = file->ha_multi_range_read_next((char **)&rec_ptr))) {
get_record_by_pos(rec_ptr);
rc = generate_full_extensions(rec_ptr);
if (rc != NESTED_LOOP_OK) return rc;
}
}
mysql> show create table t1;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`f1` int(11) DEFAULT NULL,
`f2` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table t2;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t2 | CREATE TABLE `t2` (
`f1` int(11) NOT NULL,
`f2` int(11) NOT NULL,
`f3` char(200) DEFAULT NULL,
KEY `f1` (`f1`,`f2`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> explain SELECT /*+ BKA() */ t2.f1, t2.f2, t2.f3 FROM t1,t2 WHERE t1.f1=t2.f1 AND t2.f2 BETWEEN t1.f1 and t1.f2 and t2.f2 + 1 >= t1.f1 + 1;
+----+-------------+-------+------------+------+---------------+------+---------+-------------+------+----------+---------------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------------+------+----------+---------------------------------------------------------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using where |
| 1 | SIMPLE | t2 | NULL | ref | f1 | f1 | 4 | test1.t1.f1 | 7 | 11.11 | Using index condition; Using join buffer (Batched Key Access) |
+----+-------------+-------+------------+------+---------------+------+---------+-------------+------+----------+---------------------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
c) Batched Key Access(unique)
mysql> show create table city;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| city | CREATE TABLE `city` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`Name` char(35) NOT NULL DEFAULT '',
`Country` char(3) NOT NULL DEFAULT '',
`Population` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`ID`),
KEY `Population` (`Population`),
KEY `Country` (`Country`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table country;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| country | CREATE TABLE `country` (
`Code` char(3) NOT NULL DEFAULT '',
`Name` char(52) NOT NULL DEFAULT '',
`SurfaceArea` float(10,2) NOT NULL DEFAULT '0.00',
`Population` int(11) NOT NULL DEFAULT '0',
`Capital` int(11) DEFAULT NULL,
PRIMARY KEY (`Code`),
UNIQUE KEY `Name` (`Name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
mysql> EXPLAIN SELECT city.Name, country.Name FROM city,country WHERE city.country=country.Code AND country.Name LIKE 'L%' AND city.Population > 100000;
+----+-------------+---------+------------+-------+--------------------+---------+---------+--------------------+------+----------+--------------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+--------------------+---------+---------+--------------------+------+----------+--------------------------------------------------------------+
| 1 | SIMPLE | country | NULL | index | PRIMARY,Name | Name | 208 | NULL | 1 | 100.00 | Using where; Using index |
| 1 | SIMPLE | city | NULL | ref | Population,Country | Country | 12 | test1.country.Code | 1 | 100.00 | Using where; Using join buffer (Batched Key Access (unique)) |
+----+-------------+---------+------------+-------+--------------------+---------+---------+--------------------+------+----------+--------------------------------------------------------------+
2 rows in set, 1 warning (0.01 sec)
