mysql information_schema查询大小写

mysql> select table_schema,table_name from information_schema.tables where table_name = 'test1' ;
+-----------------+------------+
| table_schema    | table_name |
+-----------------+------------+
| workoperations1 | test1      |
| workoperations2 | test1      |
| workoperations3 | test1      |
| workoperations4 | test1      |
+-----------------+------------+
4 rows in set

mysql> select table_schema,table_name from information_schema.tables where table_name = 'TEST1' ;
Empty set

若改为

select * from information_schema.tables where table_name = 'test1' ;
select * from information_schema.tables where table_name = 'TEST1' ;

都能查出4条记录。

 

原因在于

mysql> explain select table_schema,table_name from information_schema.tables where table_name = 'TEST1' ;
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
| id | select_type | table  | type | possible_keys | key        | key_len | ref  | rows | Extra                                            |
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
|  1 | SIMPLE      | tables | ALL  | NULL          | TABLE_NAME | NULL    | NULL | NULL | Using where; Skip_open_table; Scanned 1 database |
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
1 row in set

mysql> explain select * from information_schema.tables where table_name = 'TEST1' ;
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
| id | select_type | table  | type | possible_keys | key        | key_len | ref  | rows | Extra                                            |
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
|  1 | SIMPLE      | tables | ALL  | NULL          | TABLE_NAME | NULL    | NULL | NULL | Using where; Open_full_table; Scanned 1 database |
+----+-------------+--------+------+---------------+------------+---------+------+------+--------------------------------------------------+
1 row in set

两条语句的执行计划不同。
要避免出现大小写导致查不到数据字典的情况还是用*代替table_name。

附

http://dev.mysql.com/doc/refman/5.1/en/information-schema-optimization.html

TABLE_CATALOG	SKIP_OPEN_TABLE
TABLE_SCHEMA	SKIP_OPEN_TABLE
TABLE_NAME	SKIP_OPEN_TABLE
TABLE_TYPE	OPEN_FRM_ONLY
ENGINE	OPEN_FRM_ONLY
VERSION	OPEN_FRM_ONLY
ROW_FORMAT	OPEN_FULL_TABLE
TABLE_ROWS	OPEN_FULL_TABLE
AVG_ROW_LENGTH	OPEN_FULL_TABLE
DATA_LENGTH	OPEN_FULL_TABLE
MAX_DATA_LENGTH	OPEN_FULL_TABLE
INDEX_LENGTH	OPEN_FULL_TABLE
DATA_FREE	OPEN_FULL_TABLE
AUTO_INCREMENT	OPEN_FULL_TABLE
CREATE_TIME	OPEN_FULL_TABLE
UPDATE_TIME	OPEN_FULL_TABLE
CHECK_TIME	OPEN_FULL_TABLE
TABLE_COLLATION	OPEN_FRM_ONLY
CHECKSUM	OPEN_FULL_TABLE
CREATE_OPTIONS	OPEN_FRM_ONLY
TABLE_COMMENT	OPEN_FRM_ONLY