MySQL explain分析執行計劃
阿新 • • 發佈：2022-04-04
首先表結構為：
mysql> show create table film \G; *************************** 1. row *************************** Table: film Create Table: CREATE TABLE `film` ( `film_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT, `title` varchar(128) NOT NULL, `description` text, `release_year` year(4) DEFAULT NULL, `language_id` tinyint(3) unsigned NOT NULL, `original_language_id` tinyint(3) unsigned DEFAULT NULL, `rental_duration` tinyint(3) unsigned NOT NULL DEFAULT '3', `rental_rate` decimal(4,2) NOT NULL DEFAULT '4.99', `length` smallint(5) unsigned DEFAULT NULL, `replacement_cost`decimal(5,2) NOT NULL DEFAULT '19.99', `rating` enum('G','PG','PG-13','R','NC-17') DEFAULT 'G', `special_features` set('Trailers','Commentaries','Deleted Scenes','Behind the Scenes') DEFAULT NULL, `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, PRIMARYKEY (`film_id`), KEY `idx_title` (`title`), KEY `idx_fk_language_id` (`language_id`), KEY `idx_fk_original_language_id` (`original_language_id`), CONSTRAINT `fk_film_language` FOREIGN KEY (`language_id`) REFERENCES `language` (`language_id`) ON UPDATE CASCADE, CONSTRAINT `fk_film_language_original` FOREIGN KEY (`original_language_id`) REFERENCES `language` (`language_id`) ON UPDATE CASCADE ) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8mb4 1 row in set (0.01 sec)
解釋查詢語句：
mysql> explain select * from film where rating > 9\G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: film partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 1000 filtered: 33.33 Extra: Using where 1 row in set, 1 warning (0.01 sec)
- select_type： 常見取值SIMPLE（簡單表，即不使用表連線和子查詢），PRIMARY（主查詢，即外層查詢），UNION（UNIION第二個及後面的查詢），SUBQUERY（子查詢中第一個select）
- table：表名
- type：查詢方式，一般有ALL,index,range,ref,eq_ref,const,system,NULL（從左往右，執行效率從差到好）
- type=ALL，全表掃描，不使用條件查詢，或查詢條件非索引欄位，上面例子即為全表掃描
- type=index，索引全掃描，MySQL匹配整個索引來查詢匹配的行
mysql> explain select title from film\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: film partitions: NULL type: index possible_keys: NULL key: idx_title key_len: 514 ref: NULL rows: 1000 filtered: 100.00 Extra: Using index 1 row in set, 1 warning (0.01 sec)
- type=range，索引範圍掃描，常見於<,<=,>,>=,between等操作
mysql> explain select * from payment where customer_id >= 300 and customer_id <= 350\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: payment
partitions: NULL
type: range
possible_keys: idx_fk_customer_id
key: idx_fk_customer_id
key_len: 2
ref: NULL
rows: 1350
filtered: 100.00
Extra: Using index condition
1 row in set, 1 warning (0.01 sec) - type=ref，使用非唯一索引掃描或唯一索引的字首掃描，返回某個單值的記錄行
mysql> explain select * from payment where customer_id=350\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: payment partitions: NULL type: ref possible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: const rows: 23 filtered: 100.00 Extra: NULL 1 row in set, 1 warning (0.01 sec)
或在關聯查詢中，使用外來鍵連線
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 partitions: NULL type: ALL possible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 599 filtered: 100.00 Extra: NULL *************************** 2. row *************************** id: 1 select_type: SIMPLE table: a partitions: NULL type: ref possible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: sakila.b.customer_id rows: 26 filtered: 100.00 Extra: NULL 2 rows in set, 1 warning (0.01 sec)
- type=eq_ref，類似ref，區別是使用的索引是唯一索引，對於每個索引鍵值，表中只有一條記錄匹配；簡單地說就是多表連線中使用primary key或者unique index作為關聯條件
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 partitions: NULL type: ALL possible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 1000 filtered: 100.00 Extra: NULL *************************** 2. row *************************** id: 1 select_type: SIMPLE table: a partitions: NULL type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 2 ref: sakila.b.film_id rows: 1 filtered: 100.00 Extra: Using where 2 rows in set, 1 warning (0.01 sec)
- type=const/system，單表最多有一行匹配，查詢起來非常迅速，例如根據主鍵或唯一索引查詢
mysql> explain select * from payment where payment_id = 30\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: payment partitions: NULL type: const possible_keys: PRIMARY key: PRIMARY key_len: 2 ref: const rows: 1 filtered: 100.00 Extra: NULL 1 row in set, 1 warning (0.01 sec)
- type=NULL表示不用訪問表或索引，直接就能返回
mysql> explain select 1 from dual where 1\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: NULL partitions: NULL type: NULL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: NULL filtered: NULL Extra: No tables used 1 row in set, 1 warning (0.01 sec)
- possible_keys：表示查詢時可能會用到的索引
- keys：表示查詢時實際用到的索引
- key_len：索引欄位的長度
- rows：掃描的行數，行數大小影響查詢效率
- Extra：執行情況的說明和描述