索引失效
https://mp.weixin.qq.com/s/mwME3qukHBFul57WQLkOYg
- 未遵循最左匹配原則
- 索引列上使用了函式
- 例如,select a from table where length(b) = 5,不會走b上的索引
- 索引列上使用了計算
- 例如,select a from table where lb-1 = 5,不會走b上的索引
- 使用like%
- 例如,SELECT * FROM products WHERE products.prod_name like '%Li';不會走索引
- 但是,SELECT * FROM products WHERE products.prod_name like 'Li%';就會走索引
- 使用 OR 導致索引失效
- 例如,select a,b from table where a = 1 or b = 2,如果只在a上建立了索引,那麼這條語句不會走索引,如果想要走索引,需要建立a,b的聯合索引。
- in /not in 和 exists/ not exists使用不當
- In 不是一定會造成全表掃描的,IN 肯定會走索引,但是當
IN 的取值範圍較大時
會導致索引失效,走全表掃描
- In 不是一定會造成全表掃描的,IN 肯定會走索引,但是當
- order by使用不當
SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`sname` varchar(20) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`s_code` int(100) NULL DEFAULT NULL,
`address` varchar(100) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`height` double NULL DEFAULT NULL,
`classid` int(11) NULL DEFAULT NULL,
`create_time` datetime(0) NOT NULL ON UPDATE CURRENT_TIMESTAMP(0),
PRIMARY KEY (`id`) USING BTREE,
INDEX `普通索引`(`height`) USING BTREE,
INDEX `聯合索引`(`sname`, `s_code`, `address`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES (1, '學生1', 1, '上海', 170, 1, '2022-11-02 20:44:14');
INSERT INTO `student` VALUES (2, '學生2', 2, '北京', 180, 2, '2022-11-02 20:44:16');
INSERT INTO `student` VALUES (3, '變成派大星', 3, '京東', 185, 3, '2022-11-02 20:44:19');
INSERT INTO `student` VALUES (4, '學生4', 4, '聯通', 190, 4, '2022-11-02 20:44:25');
explain select create_time from student where sname = "變成派大星" ; -- 走索引
explain select create_time from student where s_code = 1; -- 不走索引
explain select create_time from student where address = "上海"; -- 不走索引
explain select create_time from student where address = "上海" and s_code = 1; -- 不走索引
explain select create_time from student where address = "上海" and sname = "變成派大星"; -- 走索引,這一條會被最佳化,先查sname再查address
explain select create_time from student where sname = "變成派大星" and address = "上海"; -- 走索引
explain select create_time from student where sname = "變成派大星" and s_code = 1 and address = "上海"; -- 走索引