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今天我们来讲讲如何优化MySQL的性能，主要从索引方面优化。下期文章讲讲"},{"type":"codeinline","content":[{"type":"text","text":"MySQL慢查询日志"}]},{"type":"text","text":"，我们是依据慢查询日志来判断哪条SQL语句有问题，然后在进行优化，敬请期待"},{"type":"codeinline","content":[{"type":"text","text":"MySQL慢查询日志篇"}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"建表"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/65/652c54cad31c38977a27e2b7cac41745.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化一：全部用到索引"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"介绍"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 建立的复合索引包含了几个字段，查询的时候最好能全部用到，而且严格按照索引顺序，这样查询效率是最高的。（最理想情况，具体情况具体分析）"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 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null、or"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"函数，如：sum()、round()等等"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"手动/自动类型转换，如：id = \"1\"，本来是数字，给写成字符串了"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/26/26383fc029c545b210290bf8aa1d464f.webp","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化四：索引不要放在范围查询右边"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"举例"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":" 比如复合索引：a->b->c，当 where a=\"\" and b>10 and 3=\"\"，这时候只能用到 a 和 b，c 用不到索引，因为在范围之后索引都失效（和 B+树结构有关）"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 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模糊搜索"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"失效情况"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"like \"%张三%\""}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"like \"%张三\""}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"解决方案"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用复合索引，即 like 字段是 select 的查询字段，如：select name from table where name like \"%张三%\""}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用 like \"张三%\""}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化七：order by 优化"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 当查询语句中使用 order by 进行排序时，如果没有使用索引进行排序，会出现 filesort 文件内排序，这种情况在数据量大或者并发高的时候，会有性能问题，需要优化。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"filesort 出现的情况举例"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 字段不是索引字段"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 字段是索引字段，但是 select 中没有使用覆盖索引，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select * from staffs order by age asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 中同时存在 ASC 升序排序和 DESC 降序排序，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select a, b from staffs order by a desc, b asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 多个字段排序时，不是按照索引顺序进行 order by，即不是按照最左前缀法则，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select a, b from staffs order by b asc, a asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"索引层面解决方法"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"使用主键索引排序"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"按照最左前缀法则，并且使用覆盖索引排序，多个字段排序时，保持排序方向一致"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"在 SQL 语句中强制指定使用某索引，force index(索引名字)"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"不在数据库中排序，在代码层面排序"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"order by 排序算法"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"双路排序Mysql4.1 之前是使用双路排序，字面的意思就是两次扫描磁盘，最终得到数据，读取行指针和 ORDER BY 列，对他们进行排序，然后扫描已经排好序的列表，按照列表中的值重新从列表中读取对数据输出。也就是从磁盘读取排序字段，在 buffer 进行排序，再从磁盘读取其他字段。"}]}]}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件的磁盘 IO 非常耗时的，所以在 Mysql4.1 之后，出现了第二种算法，就是单路排序。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"单路排序从磁盘读取查询需要的所有列，按照 orderby 列在 buffer 对它们进行排序，然后扫描排序后的列表进行输出， 它的效率更快一些，避免了第二次读取数据，并且把随机 IO 变成顺序 IO，但是它会使用更多的空间， 因为它把每一行都保存在内存中了。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 当我们无可避免要使用排序时，索引层面没法在优化的时候又该怎么办呢？尽可能让 MySQL 选择使用第二种单路算法来进行排序。这样可以减少大量的随机 IO 操作,很大幅度地提高排序工作的效率。下面看看单路排序优化需要注意的点"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"单路排序优化点"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"增大 max_length_for_sort_data"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"去掉不必要的返回字段，避免select *"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"增大 sort_buffer_size 参数设置"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是sort_buffer_size 不是越大越好："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Sort_Buffer_Size 是一个 connection 级参数,在每个 connection 第一次需要使用这个 buffer 的时候,一次性分配设置的内存。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Sort_Buffer_Size 并不是越大越好,由于是 connection 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MySQL-技术专题-性能优化—索引篇
{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"前言"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 今天我们来讲讲如何优化MySQL的性能，主要从索引方面优化。下期文章讲讲"},{"type":"codeinline","content":[{"type":"text","text":"MySQL慢查询日志"}]},{"type":"text","text":"，我们是依据慢查询日志来判断哪条SQL语句有问题，然后在进行优化，敬请期待"},{"type":"codeinline","content":[{"type":"text","text":"MySQL慢查询日志篇"}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"建表"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/65/652c54cad31c38977a27e2b7cac41745.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化一：全部用到索引"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"介绍"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 建立的复合索引包含了几个字段，查询的时候最好能全部用到，而且严格按照索引顺序，这样查询效率是最高的。（最理想情况，具体情况具体分析）"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 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无效"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"b->c：b、c 都无效"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"c：c 无效"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 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null、or"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"函数，如：sum()、round()等等"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"手动/自动类型转换，如：id = \"1\"，本来是数字，给写成字符串了"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/26/26383fc029c545b210290bf8aa1d464f.webp","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化四：索引不要放在范围查询右边"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"举例"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":" 比如复合索引：a->b->c，当 where a=\"\" and b>10 and 3=\"\"，这时候只能用到 a 和 b，c 用不到索引，因为在范围之后索引都失效（和 B+树结构有关）"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c2/c2bbcabdca2d3b1ba71ae5eef7e6ff30.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化五：减少 select * 的使用"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"使用覆盖索引"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"即：select 查询字段和 where 中使用的索引字段一致。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a6/a6c192fb92820af5f7a006d41fb692e9.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化六：like 模糊搜索"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"失效情况"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"like \"%张三%\""}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"like \"%张三\""}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"解决方案"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用复合索引，即 like 字段是 select 的查询字段，如：select name from table where name like \"%张三%\""}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用 like \"张三%\""}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"SQL 案例"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化七：order by 优化"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 当查询语句中使用 order by 进行排序时，如果没有使用索引进行排序，会出现 filesort 文件内排序，这种情况在数据量大或者并发高的时候，会有性能问题，需要优化。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"filesort 出现的情况举例"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 字段不是索引字段"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 字段是索引字段，但是 select 中没有使用覆盖索引，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select * from staffs order by age asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 中同时存在 ASC 升序排序和 DESC 降序排序，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select a, b from staffs order by a desc, b asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"order by 多个字段排序时，不是按照索引顺序进行 order by，即不是按照最左前缀法则，如："},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"select a, b from staffs order by b asc, a asc;"}],"marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}]}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"索引层面解决方法"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"使用主键索引排序"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"按照最左前缀法则，并且使用覆盖索引排序，多个字段排序时，保持排序方向一致"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"在 SQL 语句中强制指定使用某索引，force index(索引名字)"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"不在数据库中排序，在代码层面排序"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"order by 排序算法"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"双路排序Mysql4.1 之前是使用双路排序，字面的意思就是两次扫描磁盘，最终得到数据，读取行指针和 ORDER BY 列，对他们进行排序，然后扫描已经排好序的列表，按照列表中的值重新从列表中读取对数据输出。也就是从磁盘读取排序字段，在 buffer 进行排序，再从磁盘读取其他字段。"}]}]}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件的磁盘 IO 非常耗时的，所以在 Mysql4.1 之后，出现了第二种算法，就是单路排序。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"单路排序从磁盘读取查询需要的所有列，按照 orderby 列在 buffer 对它们进行排序，然后扫描排序后的列表进行输出， 它的效率更快一些，避免了第二次读取数据，并且把随机 IO 变成顺序 IO，但是它会使用更多的空间， 因为它把每一行都保存在内存中了。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 当我们无可避免要使用排序时，索引层面没法在优化的时候又该怎么办呢？尽可能让 MySQL 选择使用第二种单路算法来进行排序。这样可以减少大量的随机 IO 操作,很大幅度地提高排序工作的效率。下面看看单路排序优化需要注意的点"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"单路排序优化点"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"增大 max_length_for_sort_data"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"去掉不必要的返回字段，避免select *"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"增大 sort_buffer_size 参数设置"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是sort_buffer_size 不是越大越好："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Sort_Buffer_Size 是一个 connection 级参数,在每个 connection 第一次需要使用这个 buffer 的时候,一次性分配设置的内存。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Sort_Buffer_Size 并不是越大越好,由于是 connection 级的参数,过大的设置和高并发可能会耗尽系统内存资源。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"据说 Sort_Buffer_Size 超过 2M 的时候,就会使用 mmap() 而不是 malloc() 来进行内存分配,导致效率降低。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"优化八：group by"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 其原理也是先排序后分组，其优化方式可参考order by。where高于having,能写在where限定的条件就不要去"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"having"},{"type":"text","text":"限定了。"}]}]}
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