mysql优化之前缀索引--带案例分析

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原创 最新推荐文章于 2026-03-21 01:13:14 发布 1.4k 阅读 AI 写同款· GEO 优化 ›
MySQL 索引优化 MySQL 5.6引入的索引下推（Index Condition Pushdown）技术可以部分优化不符合最左前缀的查询。当创建复合索引时，MySQL会按照索引定义的字段顺序，从左到右使用这些字段来构建索引结构。不在最左前缀中，但索引下推可以在索引扫描时过滤。条件，减少回表次数，提高性能。 阅读详情

为什么要有前缀索引

有时候需要索引很长的字符串，这会让索引变的大且慢，通常情况下可以使用某个列开始的部分字符串，这样大大的节约索引空间，从而提高索引效率，但这会降低索引的选择性，索引的选择性是指不重复的索引值和数据表记录总数的比值，范围从1/#T到1之间。索引的选择性越高则查询效率越高，因为选择性更高的索引可以让mysql在查找的时候过滤掉更多的行。

一般情况下某个列前缀的选择性也是足够高的，足以满足查询的性能，但是对应BLOB,TEXT,VARCHAR类型的列，必须要使用前缀索引，因为mysql不允许索引这些列的完整长度，使用该方法的诀窍在于要选择足够长的前缀以保证较高的选择性，通过又不能太长。

案例：

说明：用到的city表是mysql官网上的sakila，可以直接上官网上下载然后导入，(也可以点我下载），也可以移到文章最下面我把建表语句跟插入语句提供给大家

创建数据表，插入数据

mysql> create table citydemo(city varchar(50) not null);
Query OK, 0 rows affected (0.32 sec)

mysql> insert into citydemo(city) select city from city;

重复执行5次下面的sql语句

mysql> insert into citydemo(city) select city from citydemo;
insert into citydemo(city) select city from citydemo;
insert into citydemo(city) select city from citydemo;
insert into citydemo(city) select city from citydemo;
insert into citydemo(city) select city from citydemo;

更新城市表的名称，随机更新

mysql> update citydemo set city=(select city from city order by rand() limit 1);
Query OK, 19164 rows affected (9.30 sec)

查找最常见的城市列表，发现每个值都出现45-65次，

mysql> select count(*) as cnt,city from citydemo group by city order by cnt desc limit 10;
+-----+----------------------+
| cnt | city                 |
+-----+----------------------+
|  66 | London               |
|  53 | Buenaventura         |
|  48 | Cam Ranh             |
|  47 | Funafuti             |
|  47 | Mandi Bahauddin      |
|  46 | Shivapuri            |
|  46 | San Miguel de Tucumn |
|  46 | Novi Sad             |
|  46 | San Lorenzo          |
|  46 | Kanchrapara          |
+-----+----------------------+
10 rows in set (0.02 sec)

查找最频繁出现的城市前缀，先从3个前缀字母开始，发现比原来出现的次数更多很多，一直往下加，可以分别截取多个字符查看城市出现的次数，加到7左右发现前缀的选择性接近于完整列的选择性

mysql> select count(*) as cnt,left(city,3) as pref from citydemo group by pref order by cnt desc limit 10;
+-----+------+
| cnt | pref |
+-----+------+
| 465 | San  |
| 185 | Cha  |
| 167 | Tan  |
| 166 | Sal  |
| 156 | al-  |
| 154 | Sou  |
| 148 | Man  |
| 147 | Shi  |
| 132 | Val  |
| 131 | Bat  |
+-----+------+
10 rows in set (0.02 sec)

mysql> select count(*) as cnt,left(city,7) as pref from citydemo group by pref order by cnt desc limit 10;
+-----+---------+
| cnt | pref    |
+-----+---------+
|  74 | San Fel |
|  66 | London  |
|  58 | Valle d |
|  55 | Santiag |
|  53 | Buenave |
|  48 | Cam Ran |
|  47 | Mandi B |
|  47 | Funafut |
|  46 | Novi Sa |
|  46 | San Mig |
+-----+---------+
10 rows in set (0.02 sec)

可以通过下面这种方式来计算完整列的选择性，可以看到当前缀长度到达7之后，再增加前缀长度，选择性提升的幅度已经很小了，所以选择7

mysql> select count(distinct left(city,3))/count(*) as sel3,
    -> count(distinct left(city,4))/count(*) as sel4,
    -> count(distinct left(city,5))/count(*) as sel5,
    -> count(distinct left(city,6))/count(*) as sel6,
    -> count(distinct left(city,7))/count(*) as sel7,
    -> count(distinct left(city,8))/count(*) as sel8
    -> from citydemo;
+--------+--------+--------+--------+--------+--------+
| sel3   | sel4   | sel5   | sel6   | sel7   | sel8   |
+--------+--------+--------+--------+--------+--------+
| 0.0239 | 0.0293 | 0.0305 | 0.0309 | 0.0310 | 0.0310 |
+--------+--------+--------+--------+--------+--------+
1 row in set (0.06 sec)

计算完成之后创建前缀索引

mysql> alter table citydemo add key(city(7));
Query OK, 0 rows affected, 1 warning (0.70 sec)
Records: 0  Duplicates: 0  Warnings: 1

注意：前缀索引是一种能使索引更小更快的有效方法，但是也包含缺点：mysql无法使用前缀索引做order by 和 group by。

 

city表结构数据：

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 DELETE RESTRICT ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=601 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;


INSERT INTO `city` VALUES (1, 'A Corua (La Corua)', 87, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (2, 'Abha', 82, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (3, 'Abu Dhabi', 101, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (4, 'Acua', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (5, 'Adana', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (6, 'Addis Abeba', 31, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (7, 'Aden', 107, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (8, 'Adoni', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (9, 'Ahmadnagar', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (10, 'Akishima', 50, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (11, 'Akron', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (12, 'al-Ayn', 101, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (13, 'al-Hawiya', 82, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (14, 'al-Manama', 11, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (15, 'al-Qadarif', 89, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (16, 'al-Qatif', 82, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (17, 'Alessandria', 49, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (18, 'Allappuzha (Alleppey)', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (19, 'Allende', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (20, 'Almirante Brown', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (21, 'Alvorada', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (22, 'Ambattur', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (23, 'Amersfoort', 67, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (24, 'Amroha', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (25, 'Angra dos Reis', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (26, 'Anpolis', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (27, 'Antofagasta', 22, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (28, 'Aparecida de Goinia', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (29, 'Apeldoorn', 67, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (30, 'Araatuba', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (31, 'Arak', 46, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (32, 'Arecibo', 77, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (33, 'Arlington', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (34, 'Ashdod', 48, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (35, 'Ashgabat', 98, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (36, 'Ashqelon', 48, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (37, 'Asuncin', 73, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (38, 'Athenai', 39, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (39, 'Atinsk', 80, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (40, 'Atlixco', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (41, 'Augusta-Richmond County', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (42, 'Aurora', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (43, 'Avellaneda', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (44, 'Bag', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (45, 'Baha Blanca', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (46, 'Baicheng', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (47, 'Baiyin', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (48, 'Baku', 10, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (49, 'Balaiha', 80, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (50, 'Balikesir', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (51, 'Balurghat', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (52, 'Bamenda', 19, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (53, 'Bandar Seri Begawan', 16, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (54, 'Banjul', 37, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (55, 'Barcelona', 104, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (56, 'Basel', 91, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (57, 'Bat Yam', 48, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (58, 'Batman', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (59, 'Batna', 2, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (60, 'Battambang', 18, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (61, 'Baybay', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (62, 'Bayugan', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (63, 'Bchar', 2, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (64, 'Beira', 63, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (65, 'Bellevue', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (66, 'Belm', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (67, 'Benguela', 4, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (68, 'Beni-Mellal', 62, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (69, 'Benin City', 69, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (70, 'Bergamo', 49, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (71, 'Berhampore (Baharampur)', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (72, 'Bern', 91, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (73, 'Bhavnagar', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (74, 'Bhilwara', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (75, 'Bhimavaram', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (76, 'Bhopal', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (77, 'Bhusawal', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (78, 'Bijapur', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (79, 'Bilbays', 29, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (80, 'Binzhou', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (81, 'Birgunj', 66, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (82, 'Bislig', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (83, 'Blumenau', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (84, 'Boa Vista', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (85, 'Boksburg', 85, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (86, 'Botosani', 78, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (87, 'Botshabelo', 85, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (88, 'Bradford', 102, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (89, 'Braslia', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (90, 'Bratislava', 84, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (91, 'Brescia', 49, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (92, 'Brest', 34, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (93, 'Brindisi', 49, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (94, 'Brockton', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (95, 'Bucuresti', 78, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (96, 'Buenaventura', 24, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (97, 'Bydgoszcz', 76, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (98, 'Cabuyao', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (99, 'Callao', 74, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (100, 'Cam Ranh', 105, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (101, 'Cape Coral', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (102, 'Caracas', 104, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (103, 'Carmen', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (104, 'Cavite', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (105, 'Cayenne', 35, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (106, 'Celaya', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (107, 'Chandrapur', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (108, 'Changhwa', 92, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (109, 'Changzhou', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (110, 'Chapra', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (111, 'Charlotte Amalie', 106, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (112, 'Chatsworth', 85, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (113, 'Cheju', 86, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (114, 'Chiayi', 92, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (115, 'Chisinau', 61, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (116, 'Chungho', 92, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (117, 'Cianjur', 45, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (118, 'Ciomas', 45, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (119, 'Ciparay', 45, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (120, 'Citrus Heights', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (121, 'Citt del Vaticano', 41, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (122, 'Ciudad del Este', 73, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (123, 'Clarksville', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (124, 'Coacalco de Berriozbal', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (125, 'Coatzacoalcos', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (126, 'Compton', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (127, 'Coquimbo', 22, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (128, 'Crdoba', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (129, 'Cuauhtmoc', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (130, 'Cuautla', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (131, 'Cuernavaca', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (132, 'Cuman', 104, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (133, 'Czestochowa', 76, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (134, 'Dadu', 72, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (135, 'Dallas', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (136, 'Datong', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (137, 'Daugavpils', 54, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (138, 'Davao', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (139, 'Daxian', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (140, 'Dayton', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (141, 'Deba Habe', 69, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (142, 'Denizli', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (143, 'Dhaka', 12, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (144, 'Dhule (Dhulia)', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (145, 'Dongying', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (146, 'Donostia-San Sebastin', 87, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (147, 'Dos Quebradas', 24, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (148, 'Duisburg', 38, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (149, 'Dundee', 102, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (150, 'Dzerzinsk', 80, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (151, 'Ede', 67, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (152, 'Effon-Alaiye', 69, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (153, 'El Alto', 14, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (154, 'El Fuerte', 60, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (155, 'El Monte', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (156, 'Elista', 80, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (157, 'Emeishan', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (158, 'Emmen', 67, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (159, 'Enshi', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (160, 'Erlangen', 38, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (161, 'Escobar', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (162, 'Esfahan', 46, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (163, 'Eskisehir', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (164, 'Etawah', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (165, 'Ezeiza', 6, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (166, 'Ezhou', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (167, 'Faaa', 36, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (168, 'Fengshan', 92, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (169, 'Firozabad', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (170, 'Florencia', 24, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (171, 'Fontana', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (172, 'Fukuyama', 50, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (173, 'Funafuti', 99, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (174, 'Fuyu', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (175, 'Fuzhou', 23, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (176, 'Gandhinagar', 44, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (177, 'Garden Grove', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (178, 'Garland', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (179, 'Gatineau', 20, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (180, 'Gaziantep', 97, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (181, 'Gijn', 87, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (182, 'Gingoog', 75, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (183, 'Goinia', 15, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (184, 'Gorontalo', 45, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (185, 'Grand Prairie', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (186, 'Graz', 9, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (187, 'Greensboro', 103, '2006-2-15 04:45:25');
INSERT INTO `city` VALUES (188, 'Guadalajara', 60, '2006-2-15 04:45:25');
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第43讲：MySQL数据库中前缀索引的概念以及使用 选择性中不重复的索引值，是取出指定个数的字符串作为前缀后，用这一串短的字符串去统计有多少条不重复的数据，例如我们取5个字符串作为前缀，那么就查询这个5个字符串占这一列中不重复的有多少条，统计出来有30条不重复的，那么表中数据总数也是30，最后选择性的值就是1，此时性能将会是最高的。我们可以根据索引的选择性来决定，选择性指的是不重复的索引值的数量和数据表中记录的总数的一个比值，例如不重复的索引数为100，表中数据量也是100，那么比值就是索引数处于总数据量，最后等于1，这个1就可以作为索引的选择性。 阅读详情

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