TiDB中有两种方法管理执行计划，Hunt和SPM

表结构如下

CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  KEY `idx_a` (`a`),
  KEY `idx_ab` (`a`,`b`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
mysql> select * from t1 limit 10;
+------+--------+
| a    | b      |
+------+--------+
|    1 | 753597 |
|    2 | 249532 |
|    3 | 545404 |
|    4 | 599160 |
|    5 | 112835 |
|    6 | 358078 |
|    7 |  20667 |
|    8 | 638018 |
|    9 | 317102 |
|   10 | 300015 |
+------+--------+

默认的执行计划如下，可以看到默认使用了idx_ab，走了IndexReader，且不需要回表；

mysql> desc select * from t1 where a < 10;  
+------------------------+---------+-----------+------------------------------+-----------------------------------+
| id                     | estRows | task      | access object                | operator info                     |
+------------------------+---------+-----------+------------------------------+-----------------------------------+
| IndexReader_6          | 3323.33 | root      |                              | index:IndexRangeScan_5            |
| └─IndexRangeScan_5     | 3323.33 | cop[tikv] | table:t1, index:idx_ab(a, b) | range:[-inf,10), keep order:false |
+------------------------+---------+-----------+------------------------------+-----------------------------------+
2 rows in set (0.00 sec)

使用Hint方法强制要求使用idx_a索引；

mysql> desc select * from t1 use index(idx_a) where a < 10;
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
| id                            | estRows | task      | access object            | operator info                     |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
| IndexLookUp_7                 | 9.00    | root      |                          |                                   |
| ├─IndexRangeScan_5(Build)     | 9.00    | cop[tikv] | table:t1, index:idx_a(a) | range:[-inf,10), keep order:false |
| └─TableRowIDScan_6(Probe)     | 9.00    | cop[tikv] | table:t1                 | keep order:false                  |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
3 rows in set (0.00 sec)

使用SPM方法

mysql> create binding for
    -> select * from t1 where a < 10
    -> using
    -> select * from  t1 use index(idx_a) where a < 10;
Query OK, 0 rows affected (0.00 sec)
mysql> desc select * from t1 where a < 10;
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
| id                            | estRows | task      | access object            | operator info                     |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
| IndexLookUp_7                 | 9.00    | root      |                          |                                   |
| ├─IndexRangeScan_5(Build)     | 9.00    | cop[tikv] | table:t1, index:idx_a(a) | range:[-inf,10), keep order:false |
| └─TableRowIDScan_6(Probe)     | 9.00    | cop[tikv] | table:t1                 | keep order:false                  |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------+
3 rows in set (0.00 sec)

可以看到，使用了BINDING后，即使不适用Hint，也会强制走索引idx_a

查看使用的SPM计划

mysql> show bindings\G
*************************** 1. row ***************************
Original_sql: select * from `test` . `t1` where `a` < ?
    Bind_sql: SELECT * FROM `TEST`.`t1` USE INDEX (`idx_a`) WHERE `a` < 10
  Default_db: test
      Status: using
 Create_time: 2022-01-23 17:52:17.468
 Update_time: 2022-01-23 17:52:17.468
     Charset: utf8
   Collation: utf8_general_ci
      Source: manual
1 row in set (0.00 sec)

mysql> 
mysql> --删除SPM计划：DROP [GLOBAL|SESSION] BINGING FOR ...
mysql> DROP session binding for SELECT * FROM `TEST`.`t1` USE INDEX (`idx_a`) WHERE `a` < 10;
Query OK, 0 rows affected (0.00 sec)

mysql> show bindings\G
Empty set (0.00 sec)

mysql>