<code class="hljs oxygene"><code class="hljs asciidoc">1、all -- 環境描述 ([email protected]) [sakila]> show variables like 'version'; +---------------+--------+ | Variable_name | Value  | +---------------+--------+ | version      | 5.6.26 | +---------------+--------+ MySQL採取全表遍歷的方式來返回資料行，等同於Oracle的full table scan ([email protected]) [sakila]> explain select count(description) from film; +----+-------------+-------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------+ |  1 | SIMPLE      | film  | ALL  | NULL          | NULL | NULL    | NULL | 1000 | NULL  | +----+-------------+-------+------+---------------+------+---------+------+------+-------+ 2、index MySQL採取索引全掃描的方式來返回資料行，等同於Oracle的full index scan (

[email protected]) [sakila]> explain select title from film \G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: film         type: index possible_keys: NULL           key: idx_title       key_len: 767           ref: NULL         rows: 1000         Extra: Using index 1 row in set (0.00 sec) 3、  range 索引範圍掃描，對索引的掃描開始於某一點，返回匹配值域的行，常見於between、<、>等的查詢 等同於Oracle的index range scan (

[email protected]) [sakila]> explain select * from payment where customer_id>300 and customer_id<400\G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: payment         type: range possible_keys: idx_fk_customer_id           key: idx_fk_customer_id       key_len: 2           ref: NULL         rows: 2637         Extra: Using where 1 row in set (0.00 sec) (

[email protected]) [sakila]> explain select * from payment where customer_id in (200,300,400)\G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: payment         type: range possible_keys: idx_fk_customer_id           key: idx_fk_customer_id       key_len: 2           ref: NULL         rows: 86         Extra: Using index condition 1 row in set (0.00 sec) 4、ref 非唯一性索引掃描或者，返回匹配某個單獨值的所有行。常見於使用非唯一索引即唯一索引的非唯一字首進行的查詢 ([email protected]) [sakila]> explain select * from payment where customer_id=305\G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: payment         type: ref possible_keys: idx_fk_customer_id           key: idx_fk_customer_id       key_len: 2           ref: const         rows: 25         Extra: 1 row in set (0.00 sec) idx_fk_customer_id為表payment上的外來鍵索引，且存在多個不不唯一的值，如下查詢 ([email protected]) [sakila]> select customer_id,count(*) from payment group by customer_id     -> limit 2; +-------------+----------+ | customer_id | count(*) | +-------------+----------+ |          1 |      32 | |          2 |      27 | +-------------+----------+ -- 下面是非唯一字首索引使用ref的示例 ([email protected]) [sakila]> create index idx_fisrt_last_name on customer(first_name,last_name); Query OK, 599 rows affected (0.09 sec) Records: 599  Duplicates: 0  Warnings: 0 ([email protected]) [sakila]> select first_name,count(*) from customer group by first_name     -> having count(*)>1 limit 2; +------------+----------+ | first_name | count(*) | +------------+----------+ | JAMIE      |        2 | | JESSIE    |        2 | +------------+----------+ 2 rows in set (0.00 sec) ([email protected]) [sakila]> explain select first_name from customer where first_name='JESSIE'\G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: customer         type: ref possible_keys: idx_fisrt_last_name           key: idx_fisrt_last_name       key_len: 137           ref: const         rows: 2         Extra: Using where; Using index 1 row in set (0.00 sec) ([email protected]) [sakila]> alter table customer drop index idx_fisrt_last_name; Query OK, 599 rows affected (0.03 sec) Records: 599  Duplicates: 0  Warnings: 0 --下面演示出現在join是ref的示例 ([email protected]) [sakila]> explain select b.*,a.* from payment a inner join     -> customer b on a.customer_id=b.customer_id\G *************************** 1. row ***************************           id: 1   select_type: SIMPLE         table: b         type: ALL possible_keys: PRIMARY           key: NULL       key_len: NULL           ref: NULL         rows: 599         Extra: NULL *************************** 2. row ***************************           id: 1   select_type: SIMPLE         table: a         type: ref possible_keys: idx_fk_customer_id           key: idx_fk_customer_id       key_len: 2           ref: sakila.b.customer_id         rows: 13         Extra: NULL 2 rows in set (0.01 sec) 5、eq_ref 類似於ref，其差別在於使用的索引為唯一索引，對於每個索引鍵值，表中只有一條記錄與之匹配。 多見於主鍵掃描或者索引唯一掃描。 ([email protected]) [sakila]> explain select * from film a join film_text b     -> on a.film_id=b.film_id; +----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+ | id | select_type | table | type  | possible_keys | key    | key_len | ref              | rows | Extra      | +----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+ |  1 | SIMPLE      | b    | ALL    | PRIMARY      | NULL    | NULL    | NULL            | 1000 | NULL        | |  1 | SIMPLE      | a    | eq_ref | PRIMARY      | PRIMARY | 2      | sakila.b.film_id |    1 | Using where | +----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+ ([email protected]) [sakila]> explain select title from film where film_id=5; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ |  1 | SIMPLE      | film  | const | PRIMARY      | PRIMARY | 2      | const |    1 | NULL  | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ 6、const、system： 當MySQL對查詢某部分進行優化，這個匹配的行的其他列值可以轉換為一個常量來處理。 如將主鍵或者唯一索引置於where列表中，MySQL就能將該查詢轉換為一個常量 ([email protected]) [sakila]> create table t1(id int,ename varchar(20) unique); Query OK, 0 rows affected (0.05 sec) ([email protected]) [sakila]> insert into t1 values(1,'robin'),(2,'jack'),(3,'henry'); Query OK, 3 rows affected (0.00 sec) Records: 3  Duplicates: 0  Warnings: 0 ([email protected]) [sakila]> explain select * from (select * from t1 where ename='robin')x; +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+ | id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows | Extra | +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+ |  1 | PRIMARY    | <derived2> | system | NULL          | NULL  | NULL    | NULL  |    1 | NULL  | |  2 | DERIVED    | t1        | const  | ename        | ename | 23      | const |    1 | NULL  | +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+ 2 rows in set (0.00 sec) 7、type=NULL MySQL不用訪問表或者索引就可以直接得到結果 ([email protected]) [sakila]> explain select sysdate(); +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ |  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 1 row in set (0.00 sec)</derived2></code></code>