對於MySQL執行計劃的獲取，我們可以通過explain方式來查看，explain方式看似簡單，實際上包含的內容很多，尤其是輸出結果中的type類型列。理解這些不同的類型，對於我們SQL優化舉足輕重，本文僅描述explian輸出結果中的type列，同時給出其演示。
type：
連接類型
system 表只有一行
const 表最多只有一行匹配，通用用於主鍵或者唯一索引比較時
eq_ref 每次與之前的表合並行都只在該表讀取一行，這是除了system，const之外最好的一種，
特點是使用=，而且索引的所有部分都參與join且索引是主鍵或非空唯一鍵的索引
ref 如果每次只匹配少數行，那就是比較好的一種，使用=或<=>，可以是左覆蓋索引或非主鍵或非唯一鍵
fulltext 全文搜索
ref_or_null 與ref類似，但包括NULL
index_merge 表示出現了索引合並優化(包括交集，並集以及交集之間的並集)，但不包括跨表和全文索引。
這個比較復雜，目前的理解是合並單表的范圍索引掃描（如果成本估算比普通的range要更優的話）
unique_subquery 在in子查詢中，就是value in (select...)把形如“select unique_key_column”的子查詢替換。
PS：所以不一定in子句中使用子查詢就是低效的！
index_subquery 同上，但把形如”select non_unique_key_column“的子查詢替換
range 常數值的范圍
index a.當查詢是索引覆蓋的，即所有數據均可從索引樹獲取的時候（Extra中有Using Index）；
b.以索引順序從索引中查找數據行的全表掃描（無 Using Index）；
c.如果Extra中Using Index與Using Where同時出現的話，則是利用索引查找鍵值的意思；
d.如單獨出現，則是用讀索引來代替讀行，但不用於查找
all 全表掃描
二、連接類型部分示例
1、all
-- 環境描述
(root@localhost) [sakila]> show variables like 'version';
+---------------+--------+
| Variable_name | Value |
+---------------+--------+
| version | 5.6.26 |
+---------------+--------+
MySQL采取全表遍歷的方式來返回數據行，等同於Oracle的full table scan
(root@localhost) [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采取索引全掃描的方式來返回數據行，等同於Oracle的full index scan
(root@localhost) [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
索引范圍掃描，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
(root@localhost) [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)
(root@localhost) [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前綴進行的查找
(root@localhost) [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上的外鍵索引，且存在多個不不唯一的值，如下查詢
(root@localhost) [sakila]> select customer_id,count(*) from payment group by customer_id
-> limit 2;
+-------------+----------+
| customer_id | count(*) |
+-------------+----------+
| 1 | 32 |
| 2 | 27 |
+-------------+----------+
-- 下面是非唯一前綴索引使用ref的示例
(root@localhost) [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
(root@localhost) [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)
(root@localhost) [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)
(root@localhost) [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
(root@localhost) [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。
(root@localhost) [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 |
+----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+
(root@localhost) [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
(root@localhost) [sakila]> create table t1(id int,ename varchar(20) unique);
Query OK, 0 rows affected (0.05 sec)
(root@localhost) [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
(root@localhost) [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 | | 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
(root@localhost) [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)
