數據庫實踐丨MySQL多表join分析

作者：華為云開發者社區 2020-08-06 10:25:30

數據庫 MySQL

在數據庫查詢中,往往會需要查詢多個表的數據,比如查詢會員信息同時查詢關于這個會員的訂單信息,如果分語句查詢的話,效率會很低,就需要用到join關鍵字來連表查詢了。

Join并行

Join并行1. 多表join介紹2. 多表Join的方式不使用Join buffer使用Join buffer3. Join執行流程（老執行器）

1. 多表join介紹

JOIN子句用于根據兩個或多個表之間的相關列來組合它們。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使用新執行器實現，在這里不做討論

MySQL支持的都是Nested-Loop Join，以及它的變種。

不使用Join buffer

a) Simple Nested-Loop

對r表的每一行，完整掃描s表，根據r[i]-s[i]組成的行去判斷是否滿足條件，并返回滿足條件的結果給客戶端。

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

對r表的每一行，先根據連接條件去查詢s表索引，然后回表查到匹配的數據，并返回滿足條件的結果給客戶端。

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

從r表讀取一部分數據到join cache中，當r表數據讀完或者join cache滿后，做join操作。

JOIN_CACHE_BNL::join_matching_records(){  
 do {  
   //讀取s表的每一行 
    qep_tab->table()->file->position(qep_tab->table()->record[0]);  
   //針對s的每一行，遍歷join buffer  
   for(each record in join buffer) {  
     get_record();  
     rc = generate_full_extensions(get_curr_rec());  
     //如果不符合條件，直接返回  
     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

從r表讀取一部分數據到join cache中，s表中記錄r表被連接的列的值作為索引，查詢所有符合條件的索引，然后將這些符合條件的索引排序，然后統一回表查詢記錄。

其中，對于每一個cached record，都會有一個key，通過這個key去s表掃描所需的數據。

dsmrr_fill_buffer(){  
 while((rowids_buf_cur < rowids_buf_end) &&  
       !(res = h2->handler::multi_range_read_next(&range_info))){  
   //下壓的index條件  
   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)

與Batched Key Access不同的是，r中的列是s的唯一索引，在r記錄寫入join cache的時候，會記錄一個key的hash table，僅針對不同的key去s表中查詢。（疑問，為什么只有unique的時候才能用這種方式？不是unique的話，s表中可能會掃描出多條數據，也可以用這種方式去處理，減少s表的重復掃描）。

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執行流程（老執行器）

sub_select <--------------------------------------------+  
 | -> iterator::read() // 讀一行數據                    |  
 | -> evaluate_join_record()  //檢查這行數據是否符合條件 |  
 | -> next_select() ---+                               |  
                       |                               |  
sub_select_op  <--------+                               |  
 | -> op->put_record() // 前表數據寫入join cache        |  
   | -> put_record_in_cache()                          |  
   | -> join->record()                                 |  
     | -> join_matching_records()                      |  
       | -> (qep_tab->next_select)(join, qep_tab + 1, 0) // 繼續調用next_select  
   | -> end_send()

責任編輯：龐桂玉來源： segmentfault

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數據庫實踐丨MySQL多表join分析