在oracle中可以指定的表連接的hint有很多：ordered hint 指示oracle按照from關鍵字後的表順序來進行連接；leading hint 指示查詢優化器使用指定的表作為連接的首表，即驅動表；use_nl hint指示查詢優化器使用nested loops方式連接指定表和其他行源，並且將強制指定表作為inner表。
在mysql中就有之對應的straight_join,由於mysql只支持nested loops的連接方式，所以這裡的straight_join類似oracle中的use_nl hint。mysql優化器在處理多表的關聯的時候，很有可能會選擇錯誤的驅動表進行關聯，導致了關聯次數的增加，從而使得sql語句執行變得非常的緩慢，這個時候需要有經驗的DBA進行判斷，選擇正確的驅動表，這個時候straight_join就起了作用了，下面我們來看一看使用straight_join進行優化的案例：
1．用戶實例：spxxxxxx的一條sql執行非常的緩慢，sql如下：
73871 | root | 127.0.0.1:49665 | user_app_test | Query | 500 | Sorting result | SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM test_log a,USER b WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime)
2.查看執行計劃：
mysql> explain SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM test_log a,USER b WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime); mysql> explain SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows -> FROM test_log a,USER b -> WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 -> GROUP BY DATE(practicetime)\G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a type: ALL possible_keys: ix_test_log_userid key: NULL key_len: NULL ref: NULL rows: 416782 Extra: Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: b type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 96 ref: user_app_testnew.a.userid rows: 1 Extra: Using where 2 rows in set (0.00 sec)
3.查看索引：
mysql> show index from test_log; +————–+————+————————-+————–+————-+———–+————-+———-++ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +————–+————+————————-+————–+————-+———–+————-+———-++ | test_log | 0 | ix_test_log_unique_ | 1 | unitid | A | 20 | NULL | NULL | | BTREE | | | test_log | 0 | ix_test_log_unique_ | 2 | paperid | A | 20 | NULL | NULL | | BTREE | | | test_log | 0 | ix_test_log_unique_ | 3 | qtid | A | 20 | NULL | NULL | | BTREE | | | test_log | 0 | ix_test_log_unique_ | 4 | userid | A | 400670 | NULL | NULL | | BTREE | | | test_log | 0 | ix_test_log_unique_ | 5 | serial | A | 400670 | NULL | NULL | | BTREE | | | test_log | 1 | ix_test_log_unit | 1 | unitid | A | 519 | NULL | NULL | | BTREE | | | test_log | 1 | ix_test_log_unit | 2 | paperid | A | 2023 | NULL | NULL | | BTREE | | | test_log | 1 | ix_test_log_unit | 3 | qtid | A | 16694 | NULL | NULL | | BTREE | | | test_log | 1 | ix_test_log_serial | 1 | serial | A | 133556 | NULL | NULL | | BTREE | | | test_log | 1 | ix_test_log_userid | 1 | userid | A | 5892 | NULL | NULL | | BTREE | | +————–+————+————————-+————–+————-+———–+————-+———-+——–+——+——-+
4.調整索引，A表優化采用覆蓋索引：
mysql>alter table test_log drop index ix_test_log_userid,add index ix_test_log_userid(userid,practicetime)
5.查看執行計劃：
mysql> explain SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM test_log a,USER b WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime)\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a type: index possible_keys: ix_test_log_userid key: ix_test_log_userid key_len: 105 ref: NULL rows: 388451 Extra: Using index; Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: b type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 96 ref: user_app_test.a.userid rows: 1 Extra: Using where 2 rows in set (0.00 sec)
調整後執行稍有效果，但是還不明顯，還沒有找到要害：
SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM test_log a,USER b WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime); ………………. 143 rows in set (1 min 12.62 sec)
6.執行時間仍然需要很長，時間的消耗主要耗費在Using filesort中，參與排序的數據量有38W之多，所以需要轉換驅動表；嘗試采用user表做驅動表：使用straight_join強制連接順序：
mysql> explain SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM USER b straight_join test_log a WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime)\G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: b type: ALL possible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 42806 Extra: Using where; Using temporary; Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: a type: ref possible_keys: ix_test_log_userid key: ix_test_log_userid key_len: 96 ref: user_app_test.b.userid rows: 38 Extra: Using index 2 rows in set (0.00 sec)
執行時間已經有了質的變化，降低到了2.56秒；
mysql>SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM USER b straight_join test_log a WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime); …….. 143 rows in set (2.56 sec)
7.在分析執行計劃的第一步：Using where; Using temporary; Using filesort，user表其實也可以采用覆蓋索引來避免using where的出現，所以繼續調整索引：
mysql> show index from user; +——-+————+——————+————–+————-+———–+————-+———-+——–+——+————+———+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +——-+————+——————+————–+————-+———–+————-+———-+——–+——+————+———+ | user | 0 | PRIMARY | 1 | userid | A | 43412 | NULL | NULL | | BTREE | | | user | 0 | ix_user_email | 1 | email | A | 43412 | NULL | NULL | | BTREE | | | user | 1 | ix_user_username | 1 | username | A | 202 | NULL | NULL | | BTREE | | +——-+————+——————+————–+————-+———–+————-+———-+——–+——+————+———+ 3 rows in set (0.01 sec) mysql>alter table user drop index ix_user_username,add index ix_user_username(username,isfree); Query OK, 42722 rows affected (0.73 sec) Records: 42722 Duplicates: 0 Warnings: 0 mysql>explain SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM USER b straight_join test_log a WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime); *************************** 1. row *************************** id: 1 select_type: SIMPLE table: b type: index possible_keys: PRIMARY key: ix_user_username key_len: 125 ref: NULL rows: 42466 Extra: Using where; Using index; Using temporary; Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: a type: ref possible_keys: ix_test_log_userid key: ix_test_log_userid key_len: 96 ref: user_app_test.b.userid rows: 38 Extra: Using index 2 rows in set (0.00 sec)
8.執行時間降低到了1.43秒：
mysql>SELECT DATE(practicetime) date_time,COUNT(DISTINCT a.userid) people_rows FROM USER b straight_join test_log a WHERE a.userid=b.userid AND b.isfree=0 AND LENGTH(b.username)>4 GROUP BY DATE(practicetime); 。。。。。。。 143 rows in set (1.43 sec)
