mysql 覆盖索引

参考自 cnblogs.com/chenpingzhao/p/4776981.html

概念：索引包含查询需要的所有字段，则称为覆盖索引，不需要再回表。

判断标准：explain，如果 extra列=using index，则会覆盖索引

表结构

CREATE TABLE `inventory` (
  `inventory_id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `film_id` smallint(5) unsigned NOT NULL,
  `store_id` tinyint(3) unsigned NOT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`inventory_id`),
  KEY `idx_fk_film_id` (`film_id`),
  KEY `idx_store_id_film_id` (`store_id`,`film_id`),
  CONSTRAINT `fk_inventory_film` FOREIGN KEY (`film_id`) REFERENCES `film` (`film_id`) ON UPDATE CASCADE,
  CONSTRAINT `fk_inventory_store` FOREIGN KEY (`store_id`) REFERENCES `store` (`store_id`) ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4582 DEFAULT CHARSET=utf8 |

　explain

mysql>  EXPLAIN SELECT store_id, film_id FROM sakila.inventory\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: inventory
         type: index
possible_keys: NULL
          key: idx_store_id_film_id
      key_len: 3
          ref: NULL
         rows: 4581
        Extra: Using index
1 row in set (0.03 sec)

　　未完待续