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MyISAM和InnoDB的区别

  1. MySQL默认采用的是MyISAM。
  2. MyISAM不支持事务,而InnoDB支持。InnoDB的AUTOCOMMIT默认是打开的,即每条SQL语句会默认被封装成一个事务,自动提交,这样会影响速度,所以最好是把多条SQL语句显示放在begin和commit之间,组成一个事务去提交。
  3. InnoDB 支持数据行锁定,MyISAM不支持行锁定,只支持锁定整个表。即MyISAM同一个表上的读锁和写锁是互斥 的,MyISAM并发读写时如果等待队列中既有读请求又有写请求,默认写请求的优先级高,即使读请求先到,所以MyISAM不适合于有大量查询和修改并存 的情况,那样查询进程会长时间阻塞。因为MyISAM是锁表,所以某项读操作比较耗时会使其他写进程饿死。
  4. InnoDB支持外键,MyISAM不支持。
  5. InnoDB的主键范围更大,最大是MyISAM的2倍。
  6. InnoDB 不支持全文索引,而MyISAM支持。全文索引是指对char、varchar和text中的每个词(停用词除外)建立倒排 序索引。MyISAM的全文索引其实没啥用,因为它不支持中文分词,必须由使用者分词后加入空格再写到数据表里,而且少于4个汉字的词会和停用词一样被忽 略掉。
  7. MyISAM支持GIS数据,InnoDB不支持。即MyISAM支持以下空间数据对象:Point,Line,Polygon,Surface等。
  8. 没 有where的count(*)使用MyISAM要比InnoDB快得多。因为MyISAM内置了一个计数器,count(*)时它直 接从计数器中读,而InnoDB必须扫描全表。所以在InnoDB上执行count(*)时一般要伴随where,且where中要包含主键以外的索引 列。为什么这里特别强调“主键以外”?因为InnoDB中primary index是和raw data存放在一起的,而secondary index则是单独存放,然后有个指针指向primary key。所以只是count(*)的话使用secondary index扫描更快,而primary key则主要在扫描索引同时要返回raw data时的作用较大。

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