索引覆蓋

　　是指 如果查詢的列恰好是索引的一部分,那么查詢只需要在索引文件上進行,不需要回行到磁盤再找數據.這種查詢速度非常快,稱為”索引覆蓋”,比平時的查詢少一次到磁盤讀數據的操作。(索引正好覆蓋到查詢的數據)

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例如下面:

mysql> use exam9; Database changed mysql> desc options; +----------------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------------+---------------+------+-----+---------+-------+ | optionId | varchar(40) | NO | PRI | NULL | | | questionId | varchar(40) | YES | MUL | NULL | | | optionContent | varchar(2000) | YES | | NULL | | | optionWithTag | varchar(2000) | YES | | NULL | | | optionSequence | varchar(2) | YES | | NULL | | | isAnswer | varchar(2) | YES | | NULL | | | description | varchar(300) | YES | | NULL | | +----------------+---------------+------+-----+---------+-------+ 7 rows in set (0.00 sec)mysql> reset query cache; #清空緩存 Query OK, 0 rows affected (0.00 sec)mysql> set profiling=on;　　　　#打開profiling功能 Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> select optioncontent from options where optionid='000406aa1b89461d8cfd85f b0e5d9e01'; +------------------------+ | optioncontent | +------------------------+ | 基礎工程完畢進行回填后 | +------------------------+ 1 row in set (0.03 sec)mysql> select optionid from options where optionid='000406aa1b89461d8cfd85fb0e5d 9e01'; +----------------------------------+ | optionid | +----------------------------------+ | 000406aa1b89461d8cfd85fb0e5d9e01 | +----------------------------------+ 1 row in set (0.03 sec)mysql> show profiles;　　#顯示概要信息 +----------+------------+------------------------------------------------------- ------------------------------+ | Query_ID | Duration | Query| +----------+------------+------------------------------------------------------- ------------------------------+ | 1 | 0.03480675 | select optioncontent from options where optionid='0004 06aa1b89461d8cfd85fb0e5d9e01' | | 2 | 0.03624525 | select optionid from options where optionid='000406aa1 b89461d8cfd85fb0e5d9e01' | +----------+------------+------------------------------------------------------- ------------------------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explainselect optionid from options where optionid='000406aa1b89461d8cfd8 5fb0e5d9e01'\G ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'expla inselect optionid from options where optionid='000406aa1b89461d8cfd85fb0e5d' at line 1 mysql> explain select optionid from options where optionid='000406aa1b89461d8cfd 85fb0e5d9e01'\G #explain分析語句 *************************** 1. row ***************************id: 1select_type: SIMPLEtable: optionspartitions: NULLtype: const possible_keys: PRIMARYkey: PRIMARYkey_len: 122ref: constrows: 1filtered: 100.00Extra: Using index　　#表示索引覆蓋 1 row in set, 1 warning (0.03 sec)mysql> explain select optioncontent from options where optionid='000406aa1b89461 d8cfd85fb0e5d9e01'\G　　　　#分析語句 *************************** 1. row ***************************id: 1select_type: SIMPLEtable: optionspartitions: NULLtype: const possible_keys: PRIMARYkey: PRIMARYkey_len: 122ref: constrows: 1filtered: 100.00Extra: NULL 1 row in set, 1 warning (0.03 sec)

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轉載于:https://www.cnblogs.com/qlqwjy/p/8593076.html

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