MySql性能相关的一些概念(性能tip0）_MySQL
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题首：这是最近读《高性能MySqL 第二版》记录下来的东西~
#读锁（共享锁）、写锁（排他锁）：读锁是共享的，互不阻塞，读取同一资源互不影响；写锁排他，一个写锁会阻塞其他的读写操作。
#锁定对象的粒度：表锁和行锁。
表锁：整个表加锁，当写操作时，加写锁，资源访问排他。当没有写时，加读锁，读锁互不冲突。写锁比读锁有高优先级。开销较小。
行级锁：对一行加锁，开销大，支持最大并发处理。
#事务：ACID
A原子性：一个事务是一个原子不可分的工作单元，内部的工作不会被部分执行，要么全部执行，要么根本不执行
C一致性：数据库从一种一致状态转到另一种一致状态，事务执行过程中的数据改变不会影响数据库数据。
I 隔离性：某个事务的结果只有在事务完成后才对其他事务可见。
D持久性：一个事务提交后的结果改变将是持久的，不会马上消失。
#MVCC：多版本并发控制
#几个存储引擎：
MyISAM：加表锁，在select查询时可以在同一张表完成插入（并发插入），可以基于BLOB和TEXT的前500字符进行相关索引。
InnoDB：事务引擎，适合处理大量短期事务，基于聚簇索引，不压缩索引
Memory：基于堆的，内存存储，支持哈希索引
Archive：只支持insert和select，不支持索引，缓冲了数据写操作，插入时使用zlib算法压缩，比MyISAM磁盘IO消耗少，所有 的select查询执行全表扫描，适合logging，支持行级锁。
存储引擎 MySqL版本 事务 锁粒度 主要应用 忌用 MyISAM 全部 不支持 支持并发插入的表锁 select，insert，高负载 读写并重的场合 MyISAM Merge 全部 不支持 支持并发插入的表锁 分段归档，数据仓库 许多全局查找 Memory(Heap) 全部 不支持 表锁 中间计算，静态数据查找 大型数据集，持久性存储 InnoDB 全部 支持 支持MVCC的行级锁 事务处理 无 Falcon 6.0 支持 支持MVCC的行级锁 事务处理 无 Archive 4.1 支持 支持MVCC的行级锁 日志记录，聚合分析 需要随机读取、更新、删除 CSV 4.1 不支持 表锁 日志记录，大规模加载外部数据 需要随机读取、索引 Blackhole 4.1 支持 支持MVCC的行级锁 日志记录或同步归档 除非有特别目的，否则不适合任何场合 Federated 5.0 N/A N/A 分布式数据源 除非有特别目的，否则不适合任何场合 NDB Cluster 5.0 支持 行级锁 高可靠性 大部分典型应用 PBXT 5.0 支持 支持MVCC的行级锁 事务处理，日志记录 需要聚集索引 SolidDB 5.0 支持 支持MVCC的行级锁 事务处理 无 Maria 6.x 支持 支持MVCC的行级锁 替代MyISAM 无摘自 Change Dir bitsCN.com
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