mysql 联合索引优化

 explain select Fid from t_cyou_view_records where Fanchormasterid = 20237 and Froleid = 15081 and Fouttime is null order by Fentertime desc limit 1;
+----+-------------+---------------------+------+---------------+------+---------+------+---------+-----------------------------+
| id | select_type | table               | type | possible_keys | key  | key_len | ref  | rows    | Extra                       |
+----+-------------+---------------------+------+---------------+------+---------+------+---------+-----------------------------+
|  1 | SIMPLE      | t_cyou_view_records | ALL  | NULL          | NULL | NULL    | NULL | 1709800 | Using where; Using filesort |
+----+-------------+---------------------+------+---------------+------+---------+------+---------+-----------------------------+

看了where条件中两个字段的基数，建立联合索引会有指数级提升
show index from t_cyou_view_records
    -> ;
+---------------------+------------+-----------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+--
| Table               | Non_unique | Key_name  | Seq_in_index | Column_name     | Collation | Cardinality | Sub_part | Packed | Null | Index_type | C
+---------------------+------------+-----------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+--
| t_cyou_view_records |          0 | PRIMARY   |            1 | Fid             | A         |     1710010 |     NULL | NULL   |      | BTREE      |  
| t_cyou_view_records |          1 | idx_tcvr  |            1 | Fanchormasterid | A         |         132 |     NULL | NULL   | YES  | BTREE      |  
| t_cyou_view_records |          1 | idx_tcvrf |            1 | Froleid         | A         |       58965 |     NULL | NULL   | YES  | BTREE      |  
+---------------------+------------+-----------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+--
alter table t_cyou_view_records add index idx_tcvrf(Froleid,Fanchormasterid);
Query OK, 0 rows affected (7.79 sec)
Records: 0  Duplicates: 0  Warnings: 0

explain select Fid from t_cyou_view_records where Fanchormasterid = 20237 and Froleid = 15081 and Fouttime is null order by Fentertime desc limit 1;
+----+-------------+---------------------+------+---------------+-----------+---------+-------------+------+-----------------------------+
| id | select_type | table               | type | possible_keys | key       | key_len | ref         | rows | Extra                       |
+----+-------------+---------------------+------+---------------+-----------+---------+-------------+------+-----------------------------+
|  1 | SIMPLE      | t_cyou_view_records | ref  | idx_tcvrf     | idx_tcvrf | 18      | const,const |    3 | Using where; Using filesort |
+----+-------------+---------------------+------+---------------+-----------+---------+-------------+------+-----------------------------+

扫描行数降低为3

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