mysql index hint 在index不存在時的處理

關於index_hint

   在mysql查詢語句中能夠經過指定index_hint來告訴優化器如何使用索引，詳細能夠參考這裏

index_hint:
    USE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
  | IGNORE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
  | FORCE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

 

問題

  查詢語句中使用index_hint時，若是index_hint指定的索引不存在，那麼服務器會返回錯誤ERROR 1176 (42000)。能夠看下面的例子

drop table t1;
create table t1(id int auto_increment, a char(2), primary key (id), key idx1(a)) engine=innodb;
insert into t1 values (1,'ab');
select * from t1 force index (idx1) where a='ab';
+----+------+
| id | a    |
+----+------+
|  1 | ab   |
+----+------+
1 row in set (0.00 sec)
alter table t1 drop key idx1;
select * from t1 force index (idx1) where a='ab';
ERROR 1176 (42000): Key 'idx1' doesn't exist in table 't1'

    在實際應用中，若是查詢語句使用index_hint指定了索引，隨着業務的發展，這個索引能變得多餘了，但刪除這個索引會致使應用報錯。因而要麼保留着個無用的索引，要麼修改sql從新發布應用。這兩個作法都不太友好。

改進

  若是index_hint指定的索引不存在，服務器不返回錯誤，而是選擇報warining,那麼刪除這個索引就不會致使應用報錯。

  改進：

    新增sql_mode的類型INDEX_HINE_ERROR;

    當sql_mode設置了INDEX_HINE_ERROR類型，若是index_hint指定的索引不存在，服務器返回錯誤。

    當sql_mode沒有設置INDEX_HINE_ERROR類型，若是index_hint指定的索引不存在，服務器不返回錯誤，而是選擇報warining

  看下面的例子：

 1）sql_mode沒有設置INDEX_HINE_ERROR類型

set sql_mode='';
select * from t1 force index (idx1) where a='ab';
+----+------+
| id | a    |
+----+------+
|  1 | ab   |
+----+------+
1 row in set, 1 warning (0.00 sec)
show warnings;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1176 | Key 'idx1' doesn't exist in table 't1' |
+---------+------+----------------------------------------+
1 row in set (0.00 sec)
explain select * from t1 force index (idx1) where a='ab';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set, 1 warning (0.00 sec)
show warnings;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1176 | Key 'idx1' doesn't exist in table 't1' |
+---------+------+----------------------------------------+
1 row in set (0.00 sec)

 

 2）sql_mode設置了INDEX_HINE_ERROR類型

set sql_mode='index_hint_error';
select * from t1 force index (idx1) where a='ab';
ERROR 1176 (42000): Key 'idx1' doesn't exist in table 't1'
explain select * from t1 force index (idx1) where a='ab';
ERROR 1176 (42000): Key 'idx1' doesn't exist in table 't1'