1.explain分析
explian引用
索引基数
show indexes from table_name;
主键索引具有最好的基数
测试时
不走缓存
SELECT SQL_NO_CACHE id from test_null;
2.更好的索引类型
索引列尽可能的为not null ,避免在可空的列索引上进行二次扫描
要尽量避免 NULL ，关于索引列为Null的是否走索引，见测试 索引列的值为null查询时走索引的情况
3.使用unique index
与常规索引比不需要进行索引范围扫描
4.使用primary key
主键是uniquekey的一种特殊形式 。在innodb中,一个uniquekey是一个聚集索引（即对磁盘上数据排列的索引）,当数据按照主键的次序进行检索时会极大改进性能
5.索引太多是有害的
例如，如果possible_keys 列表中有超过3个的索引,mysql优化器有太多信息而无法确定最好使用哪个索引，也就意味着有些是低效或者无用的索引
6.索引列使用最小可能的数据类型
比如在一个varchar(100)甚至更大的列上建立索引，一种改进方法是建立一个额外的列,并在包含较大的varchar(100)列的md5值的额外varchar(32)列上创建索引。
更好的方法是使用bigint来存储md5值的数字表示,数字索引更加高效
CONV(N,from_base,to_base)
mysql> select conv(‘a‘,16,10); +-----------------+ | conv(‘a‘,16,10) | +-----------------+ | 10 | +-----------------+ mysql> select conv(substr(md5(‘abc‘),1,16),16,10); +-------------------------------------+ | conv(substr(md5(‘abc‘),1,16),16,10) | +-------------------------------------+ | 10376663631224000432 | +-------------------------------------+
7.建立索引时
如果使用到多个列,定义多列索引
哪列的唯一性更高(基数大 show indexes from table_name),哪列优先放在多列索引的前面
覆盖索引是理性的索引
覆盖索引包括所有需要的列,但是不需要读取单独的数据页,实际意味着不需要读取数据存储,只利用索引数据就可以检索到实际想要的查询的数据
在myisam表里,意味着只要读入索引就可以得到问题的记录,在innodb中 索引和数据是位于同一个文件中的,但仍然会高效些,因为只需要读入索引
优化部分索引的性能
与其在长字符的列上定义索引,还不如只在左边的一小部分上建立索引
8.一些常见的不使用索引的情况
开始字符是通配符是,或者 在索引列上使用标量函数
like "%123",upper()
9.覆盖索引的左前缀原则
10.更详细的分析
set profiling=1;
select * from table;
show profile;
show profile source ;
mysql> select * from test_null where mark like ‘aaa9999%‘; +------+---------+ | id | mark | +------+---------+ | 9999 | aaa9999 | +------+---------+ 1 row in set mysql> show profile; +----------------------+----------+ | Status | Duration | +----------------------+----------+ | starting | 5.5E-5 | | checking permissions | 1.1E-5 | | Opening tables | 2E-5 | | init | 2.4E-5 | | System lock | 7E-6 | | optimizing | 8E-6 | | statistics | 1.4E-5 | | preparing | 7E-6 | | executing | 2E-6 | | Sending data | 0.006271 | | end | 5.7E-5 | | query end | 3.6E-5 | | closing tables | 5.1E-5 | | freeing items | 0.000348 | | cleaning up | 0.00011 | +----------------------+----------+ mysql> show profile source; +----------------------+----------+-----------------------+----------------------+-------------+ | Status | Duration | Source_function | Source_file | Source_line | +----------------------+----------+-----------------------+----------------------+-------------+ | starting | 5.5E-5 | NULL | NULL | NULL | | checking permissions | 1.1E-5 | check_access | sql_authorization.cc | 835 | | Opening tables | 2E-5 | open_tables | sql_base.cc | 5648 | | init | 2.4E-5 | handle_query | sql_select.cc | 121 | | System lock | 7E-6 | mysql_lock_tables | lock.cc | 321 | | optimizing | 8E-6 | JOIN::optimize | sql_optimizer.cc | 151 | | statistics | 1.4E-5 | JOIN::optimize | sql_optimizer.cc | 367 | | preparing | 7E-6 | JOIN::optimize | sql_optimizer.cc | 475 | | executing | 2E-6 | JOIN::exec | sql_executor.cc | 119 | | Sending data | 0.006271 | JOIN::exec | sql_executor.cc | 195 | | end | 5.7E-5 | handle_query | sql_select.cc | 199 | | query end | 3.6E-5 | mysql_execute_command | sql_parse.cc | 4952 | | closing tables | 5.1E-5 | mysql_execute_command | sql_parse.cc | 5004 | | freeing items | 0.000348 | mysql_parse | sql_parse.cc | 5578 | | cleaning up | 0.00011 | dispatch_command | sql_parse.cc | 1864 | +----------------------+----------+-----------------------+----------------------+-------------+
优化update
换成select使用explain
优化delete
mysql> select * from parent; +----+------+ | id | name | +----+------+ | 1 | pa | | 2 | pb | | 3 | pc | | 4 | pd | +----+------+ 4 rows in set mysql> select * from child; +-----------+----------+ | parent_id | child_id | +-----------+----------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 1 | 4 | | 1 | 5 | | 2 | 6 | | 0 | 7 | | 5 | 8 | | 6 | 9 | | 5 | 10 | +-----------+----------+
删除child中parent_id不在parent表的记录
一般的写法是
delete from child where parent_id not in(select id from parent);
更加高效的是使用连接查询
通过以下来验证
set profiling=1;
select * from child where parent_id not in(select id from parent);
select child.* from child left join parent on child.parent_id=parent.id where parent.id is null;
select query_id,count(*) as ‘#ops‘ ,sum(duration) from information_schema.profiling group by query_id;
select * from information_schema.profiling ;
演示结果 mysql> set profiling=1; Query OK, 0 rows affected mysql> select * from child where parent_id not in(select id from parent); +-----------+----------+ | parent_id | child_id | +-----------+----------+ | 0 | 7 | | 5 | 8 | | 6 | 9 | | 5 | 10 | +-----------+----------+ 4 rows in set mysql> select child.* from child left join parent on child.parent_id=parent.id where parent.id is null; +-----------+----------+ | parent_id | child_id | +-----------+----------+ | 0 | 7 | | 5 | 8 | | 6 | 9 | | 5 | 10 | +-----------+----------+ 4 rows in set mysql> select query_id,count(*) as ‘#ops‘ ,sum(duration) from information_schema.profiling group by query_id; +----------+------+---------------+ | query_id | #ops | sum(duration) | +----------+------+---------------+ | 1 | 23 | 0.000749 | | 2 | 16 | 0.000388 | +----------+------+---------------+
优化器显示第二个用了更少的操作
优化Insert，同一表的多条类似的多个insert改写成1条减少数据库的网络往返
例外一个好处是mysql只需为insert语句产生一次执行计划,可以在多个值上利用同一个执行计划
当批量插入时,如果单个插入失败,多个value子句说明的记录都无法插入成功
优化insert ...on duplicate key update
replace在内部是使用delete和insert来实现的,因而其效率并不高
使用insert ...on duplicate key update
如果存在同样主键值的记录,而其它列与现在存指定的记录有所不同,就更新该记录,如果记录不存在就插入该记录,如果记录存在而且没有任何值发生改变
就不做任何操作,优于replace
mysql> desc a; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | sid | int(11) | YES | | NULL | | | type | char(10) | NO | | NULL | | +-------+----------+------+-----+---------+----------------+ mysql> select * from a; +----+-----+------+ | id | sid | type | +----+-----+------+ | 1 | 11 | aa | | 2 | 1 | b | | 3 | 2 | c | | 4 | 3 | d | +----+-----+------+ mysql> insert into a(`id`,`type`) values(1,‘a1‘); 1062 - Duplicate entry ‘1‘ for key ‘PRIMARY‘ mysql> insert into a(`id`,`type`) values(1,‘a1‘) on duplicate key update type=‘a1‘; Query OK, 2 rows affected 注意改变的是2行 mysql> insert into a(`id`,`type`) values(5,‘a5‘) on duplicate key update type=‘a5‘; Query OK, 1 row affected mysql> select * from a; +----+------+------+ | id | sid | type | +----+------+------+ | 1 | 11 | a1 | | 2 | 1 | b | | 3 | 2 | c | | 4 | 3 | d | | 5 | NULL | a5 | +----+------+------+
待续...
