mysql 8.0降序索引

MySQL支持降序索引：DESC在索引定义中不再被忽略，但会导致按键降序存储键值。
而且只有InnoDB存储引擎支持降序索引
例如：创建表
CREATE TABLE test.t (
id INT NOT NULL auto_increment PRIMARY KEY,c1 INT, c2 INT
);
插入数据
INSERT into test.t (c1,c2) VALUES (1,2),(1,3),(1,4),(4,2),(4,1),(4,3),(2,1),(2,3),(2,4),(3,2),(3,1),(3,4);
使用sql优化器查看
EXPLAIN select FROM test.t WHERE c1=1 ORDER BY c1 ASC, c2 ASC;

创建索引排序
CREATE INDEX idx1 ON test.t(c1,c2);
使用sql优化器查看
EXPLAIN select FROM test.t WHERE c1=1 ORDER BY c1 ASC, c2 ASC;

使用sql优化器查看
EXPLAIN select FROM test.t WHERE c1=1 ORDER BY c1 ASC, c2 DESC;

创建索引排序
CREATE INDEX idx1 ON test.t(c1,c2);
使用sql优化器查看
EXPLAIN
SELECT FROM test.t ORDER BY c1 ASC,c2 DESC;

查看表结构
SHOW CREATE TABLE t;