mysql----order by优化

order by:

create table test04(
id int primary key auto_increment,
c1 char(10),
c2 char(10),
c3 char(10),
c4 char(10),
c5 char(10)
);

insert into test04(c1,c2,c3,c4,c5)
values
('a1','a2','a3','a4','a5'),
('b1','b2','b3','b4','b5'),
('c1','c2','c3','c4','c5'),
('d1','d2','d3','d4','d5'),
('e1','e2','e3','e4','e5');

建索引：create index inx_1234 on test04(c1,c2,c3,c4);

mysql> explain select * from test04 where c1 = 'a1';
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | test04 | NULL | ref | inx_1234 | inx_1234 | 31 | const | 1 | 100.00 | NULL |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.13 sec)

mysql> explain select * from test04 where c1 = 'a1' and c2 = 'a2' and c4 > 'a4' order by c3 ;
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+-----------------------+
| 1 | SIMPLE | test04 | NULL | ref | inx_1234 | inx_1234 | 62 | const,const | 1 | 33.33 | Using index condition |
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+-----------------------+
1 row in set, 1 warning (0.05 sec)
用到了两个索引，从排序的角度理解，c3的也用到了。

mysql> explain select * from test04 where c1 = 'a1' and c2 = 'a2' order by c4 ;
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+----------------+
| 1 | SIMPLE | test04 | NULL | ref | inx_1234 | inx_1234 | 62 | const,const | 1 | 100.00 | Using filesort |
+----+-------------+--------+------------+------+---------------+----------+---------+-------------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
一定只用到了两个索引。并且出现了using filesort.

mysql> explain select * from test04 where c1 = 'a1' and c5 = 'a5' order by 'c2,c3';
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test04 | NULL | ref | inx_1234 | inx_1234 | 31 | const | 1 | 20.00 | Using where |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
用到了一个查找索引，两个排序索引。c5是干扰项。
mysql 8> explain select * from test04 where c1 = 'a1' and c5 = 'a5' order by 'c3,c2';
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test04 | NULL | ref | inx_1234 | inx_1234 | 31 | const | 1 | 20.00 | Using where |
+----+-------------+--------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
以上两个例子，第二个在低版本中会出现using filesort.
一般情况下如果order by的顺序和复合索引的顺序不一致都会出现using filesort,前提是查找列中无排序列。

order by 子句尽量使用index方式排序，避免使用filesort方式排序，且尽可能在索引上完成排序稻作，遵照索引建的最佳左前缀法则。
create table tabA(
age int,
birth timestamp not null
);
insert into tabA(age,birth) values (23,now());
insert into tabA(age,birth) values (24,now());
insert into tabA(age,birth) values (25,now());
insert into tabA(age,birth) values (26,now());

从以上两个列子可以看出虽然用到了索引，但是如果order by 的顺序和索引建的顺序不一致的时候会产生using filesort.

因为order by默认升序两个排序存在冲突，因此会产生using filesort.
此时仅仅依靠索引是无法解决的，需要修改mysql的配置文件的
sort_buffer_size的容量，同时可能也需要改max_length_for_size_data的容量。

group by的优化原则和order by差不多：
（1）分组顺序按照复合索引的最佳左前缀原则进行；
（2）当分组时无法直接使用索引时应考虑修改配置文件的sort_buffer_size，max_length_for_size_data
具体修改的大小依照具体情况而定；
（3）having语句基于group by，但是也是用于过滤，因此能在where后直接过滤的尽量不用在使用having；

CF_S

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mysql----order by优化

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