BTREE索引与HASH索引

      memory存储引擎的表可以选择使用BTREE索引或者HASH索引，分别有自己的适用范围。

如果童鞋没有文中提到的表，请看附注。

• hash索引的适用范围及缺陷
  1、只用于 = 或<=>操作符的等式比较。
  2、优化器不能使用hash索引加速order by操作；
  3、当~ mysql不能确定在两个值之间大约多少行。若将一个myisam表改为hash索引的memory表时，会影响一些查询的执行效率。
  4、只能使用整个关键字搜索一行。
• BTREE索引
  适用于 >、<、>=、<=、between、！=或者<> ,或者 like ‘parent’(parent不以通配符开始)操作符。
• 执行sql简单了解下两种索引。
  1、下面范围查找适合btree和hash索引：select * from customers where age=1 or age in(26,2,3);;
  
  2、下面范围查询只适用于BTREE索引：
  select * from customers where age >20 and age<30;
select * from customers where name like 't%' or name between 'tom' and 'jerry';
  eg：创建一个和city表完全相同的memory 存储引擎的表city_memory：

 CREATE TABLE `city_memory` (
  `city_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `city` varchar(50) NOT NULL,
  `country_id` smallint(5) unsigned NOT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON
  UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`city_id`),
  KEY `idx_fk_country_id` (`country_id`),
  CONSTRAINT `fk_city_country` FOREIGN KEY (`country_id`) REFERENCES
  `country` (`country_id`) ON UPDATE CASCADE
) ENGINE=Memory  DEFAULT CHARSET=utf8;
insert into city_memory select * from city;

当对索引字段进行范围查询的时候，只有BTREE 索引可以通过索引访问：

explain  select * from city where country_id>1 and country_id<101  \G ;

而hash索引实际上是全表扫描的：explain select * from city_memory where country_id>1 and country_id<101 \G ;

当使用memory表时，如果默认创建hash索引，则要注意sql语句的编写，确保可以使用上索引，如果一定要使用范围查询，那么在创建索引时，则应该选择创建成BTREE索引。

附注：测试数据准备：

CREATE TABLE `customers` (
  `id` int(10) NOT NULL,
  `name` varchar(20) DEFAULT NULL,
  `age` int(3) DEFAULT NULL,
  `birthday` datetime DEFAULT NULL,
  `married` smallint(1) DEFAULT NULL COMMENT '1 :0δ',
  `photo` blob COMMENT 'Ƭʹƴ',
  `remak` varchar(100) DEFAULT NULL COMMENT 'ע',
  `ctime` timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
  `sex` char(1) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (1, 'xianzi', 26, NULL, 0, NULL, NULL, '2018-10-5 20:36:07', NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (2, 'yanhua', 2, NULL, 0, NULL, NULL, '2018-10-5 20:36:10', NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (3, 'haidao', 3, NULL, 0, NULL, NULL, '2018-10-5 20:36:13', NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (4, 'xiao_lang', 14, NULL, 0, NULL, NULL, NULL, NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (5, 'yaxian_baby', 2, NULL, 0, NULL, NULL, NULL, NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (6, 'tom', 1, NULL, 0, NULL, NULL, NULL, NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (7, 'jerry', 1, NULL, 0, NULL, NULL, NULL, NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (8, '1', 11, NULL, 0, NULL, NULL, NULL, NULL);
INSERT INTO `customers` (`id`, `name`, `age`, `birthday`, `married`, `photo`, `remak`, `ctime`, `sex`) VALUES (9, '2', 21, NULL, 0, NULL, NULL, NULL, NULL);

CREATE TABLE `city` (
  `city_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `city` varchar(50) NOT NULL,
  `country_id` smallint(5) unsigned NOT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE           CURRENT_TIMESTAMP,
  PRIMARY KEY (`city_id`),
  KEY `idx_fk_country_id` (`country_id`),
  CONSTRAINT `fk_city_country` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=223 DEFAULT CHARSET=utf8;