1.MySQL查询优化技术

• 查询重用 (把查询结果缓存到内存，下次查的时候，直接在内存用,但不建议。内存同步太频繁)

• 查询重写 （语句改写）
  ​ 例如将：子查询 --> 联表查询

• 硬件层

• mysql系统层

• schema设计层

• sql层

• 架构层

2.子查询优化

CREATE TABLE t1 (a1 INT auto_increment, b1 INT, PRIMARY KEY (a1));
CREATE TABLE t2 (a2 INT auto_increment, b2 INT, PRIMARY KEY (a2));
insert into t1 values(null,1);
insert into t2 values(null,1);

2.1 优化器自动优化

SELECT * FROM t1 WHERE t1.a1<100 AND a1 IN (SELECT a2 FROM t2 WHERE t2.a2 >10);

mysql> explain SELECT * FROM t1 WHERE t1.a1<100 AND a1 IN (SELECT a2 FROM t2 WHERE t2.a2 >10);
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+--