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HomeDatabaseMysql TutorialMonty说MySQL的优化(六)_MySQL

 二十六、如何知晓MySQL解决一条查询

  运行项列命令并试图弄明白其输出:

  SHOW VARIABLES;

  SHOW COLUMNS FROM ...G

  EXPLAIN SELECT ...G

  FLUSH STATUS;

  SELECT ...;

  SHOW STATUS;

  二十七、MySQL非常不错

  日志

  在进行很多连接时,连接非常快。

  同时使用SELECT和INSERT的场合。

  在不把更新与耗时太长的选择结合时。

  在大多数选择/更新使用唯一键码时。

  在使用没有长时间冲突锁定的多个表时。

  在用大表时(MySQL使用一个非常紧凑的表格式)。

  二十八、MySQL应避免的事情

  用删掉的行更新或插入表,结合要耗时长的SELECT。

  在能放在WHERE子句中的列上用HAVING。

  不使用键码或键码不够唯一而进行JOIN。

  在不同列类型的列上JOIN。

  在不使用=匹配整个键码时使用HEAP表。

  在MySQL监控程序中忘记在UPDATE或DELETE中使用一条WHERE子句。如果想这样做,使用mysql客户程序的--i-am-a-dummy选项。

  二十九、MySQL各种锁定

  内部表锁定

  LOCK TABLES(所有表类型适用)

  GET LOCK()/RELEASE LOCK()

  页面锁定(对BDB表)

  ALTER TABLE也在BDB表上进行表锁定

  LOCK TABLES允许一个表有多个读者和一个写者。

  一般WHERE锁定具有比READ锁定高的优先级以避免让写入方干等。对于不重要的写入方,可以使用LOW_PRIORITY关键字让锁定处理器优选读取方。

  UPDATE LOW_PRIORITY SET value=10 WHERE id=10;

  三十、给MySQL更多信息以更好地解决问题的技巧 注意你总能去掉(加注释)MySQL功能以使查询可移植:

  SELECT /*! SQL_BUFFER_RESULTS */ ...

  SELECT SQL_BUFFER_RESULTS ...

  将强制MySQL生成一个临时结果集。只要所有临时结果集生成后,所有表上的锁定均被释放。这能在遇到表锁定问题时或要花很长时间将结果传给客户端时有所帮助。

  SELECT SQL_SMALL_RESULT ... GROUP BY ...

  告诉优化器结果集将只包含很少的行。

  SELECT SQL_BIG_RESULT ... GROUP BY ...

  告诉优化器结果集将包含很多行。

  SELECT STRAIGHT_JOIN ...

  强制优化器以出现在FROM子句中的次序联结表。

  SELECT ... FROM table_name [USE INDEX (index_list) | IGNORE INDEX (index_list)] table_name2

  强制MySQL使用/忽略列出的索引

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