MySQL EXPLAIN type型別說明
EXPLAIN執行計劃中type欄位分為以下幾種：
ALL
INDEX
RANGE
REF
EQ_REF
CONST,SYSTEM
NULL
自上而下，效能從最差到最好
type = ALL，全表掃描，MYSQL掃描全表來找到匹配的行
（因為film表中rating不是索引）
mysql> explain extended select * from film where rating > 9\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: film
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1024
filtered: 100.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
type = index，索引全掃描，MYSQL遍歷整個索引來查詢匹配的行。（雖然where條件中沒有用到索引，但是要取出的列title是索引包含的列，所以只要全表掃描索引即可，直接使用索引樹查詢資料）
mysql> 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: 1024
Extra: Using index
1 row in set (0.00 sec)
type = range ，索引範圍掃描，常見於<、<=、>、>=、between等操作符（因為customer_id是索引，所以只要查詢索引的某個範圍即可，通過索引找到具體的資料）
mysql> explain select * from payment where customer_id > 300 and customer_id < 350\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: 1294
Extra: Using where
1 row in set (0.01 sec)
type = ref ，使用非唯一性索引或者唯一索引的字首掃描，返回匹配某個單獨值的記錄行。
（1）使用非唯一性索引customer_id單表查詢
mysql> explain select * from payment where customer_id = 350\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: 23
Extra:
1 row in set (0.00 sec)
（2）使用非唯一性索引聯表查詢（由於customer_id在a表中不是主鍵，是普通索引（非唯一），所以是ref）
mysql> explain select b.*, a.* from payment a ,customer b where 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: 541
Extra:
*************************** 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: 14
Extra:
2 rows in set (0.00 sec)
type = eq_ref，相對於ref來說就是使用的是唯一索引，對於每個索引鍵值，只有唯一的一條匹配記錄（在聯表查詢中使用primary key或者unique key作為關聯條件）
（在film和film_text中film_id都是主鍵，即都是唯一索引）
mysql> explain select * from film a ,film_text b where a.film_id = b.film_id\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: b
type: ALL
possible_keys: PRIMARY
key: NULL
key_len: NULL
ref: NULL
rows: 1000
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: a
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 2
ref: sakila.b.film_id
rows: 1
Extra: Using where
2 rows in set (0.00 sec)
type = const/system，單表中最多隻有一條匹配行，查詢起來非常迅速，所以這個匹配行中的其他列中的值可以被優化器在當前查詢中當做常量來處理。例如根據主鍵或者唯一索引進行的查詢。
mysql> explain select * from film where film_id = 1\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: film
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 2
ref: const
rows: 1
Extra:
1 row in set (0.02 sec)
註釋：如果上表中film表中只有一行資料，那麼type就是system。
type = NULL，MYSQL不用訪問表或者索引就直接能到結果。
mysql> explain select 1 from dual where 1\G （dual是一個虛擬的表，可以直接忽略）
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: No tables used
1 row in set (0.00 sec)
mysql> select 1+1 from dual;
+-----+
| 1+1 |
+-----+
| 2 |
+-----+
1 row in set (0.05 sec)
explain extended
mysql> explain extended select sum(amount) from customer a ,payment b where 1 = 1 and a.customer_id = b.customer_id and email = '[email protected]'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: a
type: ALL
possible_keys: PRIMARY
key: NULL
key_len: NULL
ref: NULL
rows: 541
filtered: 100.00
Extra: Using where
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: b
type: ref
possible_keys: idx_fk_customer_id
key: idx_fk_customer_id
key_len: 2
ref: sakila.a.customer_id
rows: 14
filtered: 100.00
Extra:
2 rows in set, 1 warning (0.00 sec)
mysql> show warnings\G
*************************** 1. row ***************************
Level: Note
Code: 1003
Message: select sum(`sakila`.`b`.`amount`) AS `sum(amount)` from `sakila`.`customer` `a` join `sakila`.`payment` `b` where ((`sakila`.`b`.`customer_id` = `sakila`.`a`.`customer_id`) and (`sakila`.`a`.`email` = '[email protected]'))
1 row in set (0.00 sec)