摘要：在数据库查问中,往往会须要查问多个表的数据,比方查问会员信息同时查问对于这个会员的订单信息,如果分语句查问的话,效率会很低,就须要用到join关键字来连表查问了。
Join并行
Join并行1. 多表join介绍2. 多表Join的形式不应用Join buffer应用Join buffer3. Join执行流程（老执行器）
1. 多表join介绍
JOIN子句用于依据两个或多个表之间的相干列来组合它们。 例如：
Orders：
Customers：
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDateFROM OrdersINNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;2. 多表Join的形式
Hash join应用新执行器实现，在这里不做探讨
MySQL反对的都是Nested-Loop Join，以及它的变种。
不应用Join buffer
a) Simple Nested-Loop
对r表的每一行，残缺扫描s表，依据r[i]-s[i]组成的行去判断是否满足条件，并返回满足条件的后果给客户端。
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，先依据连贯条件去查问s表索引，而后回表查到匹配的数据，并返回满足条件的后果给客户端。
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)应用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。
JOIN_CACHE_BNL::join_matching_records(){ do { //读取s表的每一行 qep_tab->table()->file->position(qep_tab->table()->record[0]); //针对s的每一行，遍历join buffer for(each record in join buffer) { get_record(); rc = generate_full_extensions(get_curr_rec()); //如果不符合条件，间接返回 if (rc != NESTED_LOOP_OK) return rc; } } while(!(error = iterator->Read()))}mysql> explain select * 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; Using join buffer (Block Nested Loop) |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+2 rows in set, 1 warning (0.00 sec)b) Batched Key Access
从r表读取一部分数据到join cache中，s表中记录r表被连贯的列的值作为索引，查问所有符合条件的索引，而后将这些符合条件的索引排序，而后对立回表查问记录。
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)
与Batched Key Access不同的是，r中的列是s的惟一索引，在r记录写入join cache的时候，会记录一个key的hash table，仅针对不同的key去s表中查问。（疑难，为什么只有unique的时候能力用这种形式？不是unique的话，s表中可能会扫描出多条数据，也能够用这种形式去解决，缩小s表的反复扫描）。JOIN_CACHE_BKA_UNIQUE::join_matching_records(){ while (!(error = file->ha_multi_range_read_next((char **)&key_chain_ptr))) { do(each record in chain){ get_record_by_pos(rec_ptr); rc = generate_full_extensions(rec_ptr); if (rc != NESTED_LOOP_OK) return rc; } }}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)3. Join执行流程（老执行器）
sub_select <--------------------------------------------+ | -> iterator::read() // 读一行数据 | | -> evaluate_join_record() //查看这行数据是否符合条件 | | -> next_select() ---+ | | |sub_select_op <--------+ | | -> op->put_record() // 前表数据写入join cache | | -> put_record_in_cache() | | -> join->record() | | -> join_matching_records() | | -> (qep_tab->next_select)(join, qep_tab + 1, 0) // 持续调用next_select | -> end_send()点击关注，第一工夫理解华为云陈腐技术~