从MySQL 5.5发布看开源数据库新模式_MySQL
尽管甲骨文收购Sun交易尚在等待最终结果，业界对开源数据库MySQL的未来命运也十分担忧，但Sun的开发者依然在继续努力研发该开源数据库。他们的劳动果实之一就是最近发布的MySQL 5.5里程碑 2开发者版本。新版引入了许多新功能，其中有些功能最初是为MySQL 6.0而准备的。
最近两年以来，MySQL界一直在讨论MySQL 6.0版。最近发布的一个MySQL稳定版是5.1版，在饱受争议和一再推迟后，该版本于一年前初次亮相。
继推出MySQL 5.1版后，MySQL开发团队改变了推出新版的模式，以支持一个更迅速和更稳定的版本升级周期。此后，某些计划在6.0版中推出的功能提前亮相。
Sun发言人史蒂夫·柯瑞(Steve Curry)指出，“作为这种新模式的一部分，我们已经将许多原先为6.0版计划的功能提前在MySQL 5.5版中实现，将来肯定会有MySQL 6.0版，但是我们尚未决定它将包含哪些新升级功能。”
柯瑞表示，他相信用户会因为MySQL 5.5里程碑版中的众多新功能而感到兴奋，其中包括性能和扩展性方面的改进。
◆MySQL 5.5新增了半同步复制功能，现在MySQL用户除了使用默认的异步复制功能之外，又多了一种复制功能可供选择。
据MySQL 5.5版本声明显示，“在返回执行复制事务的会话之前，Master端将暂停执行提交，直到slave端确认它已经收到并记录复制事务。”相比之下，在异步复制功能中，Master端将直接记录事件，而无需知道slave是否真正处理了复制事务。
◆MySQL 5.5增加了改进的partitioning语法。在这个新版数据库中，有许多新的数据库分区功能选项，其中包括用户自定义的RANGE COLUMNS分区功能选项。
MySQL官方表示，“当用户在根据RANGE COLUMNS或LIST COLUMNS对表进行分区时，可以根据多列数值相应的定义ranges或lists，最高可关联16列。”
◆在MySQL 5.5里程碑版中，还新增了对SQL标准SINGAL和RESINGNAL语句的支持。SINGNAL语句和RESINGNAL语句都可确保一个错误状态能够被返回到数据库Hander。
MySQL官方表示，“没有SINGNAL，如果想让程序返回错误，必须借助于诸如故意指向一个不错在的表等迂回解决方案。而RESINGNAL既可以处理错误又能返回错误信息。否则，通过在Handler内执行一个SQL语句，激活Handler的信息将会被破坏。”
从MySQL 5.1到MySQL 5.5
就目前而言，MySQL 5.5里程碑版还是一个开发者版，还不适合用在正式的生产环境。但这并不说明Sun不希望人们试用这个新版本，柯瑞表示，MySQL 5.5测试者的工作非常重要，可以帮助Sun收到反馈意见和估算进度。
如果想从目前的5.1版升级到还处于开发版的5.5，用户无需自行摸索升级方法。柯瑞表示，“我们正在提供大量的移植资料，帮助用户完成升级过程，并鼓励他们开始熟悉新版的功能，我们不希望用户在这一过程中遇到任何麻烦。”
至于MySQL 5.5何时结束目前的开发者里程碑版而正式发布，柯瑞表示尚未最终确定。
他表示，“将来从社区、客户和内部QA获得了初步反馈意见后，我们将有更明确的想法。通过这种新里程碑版本模式，我们正努力完善数据库所有功能。初步用户反馈意见令人满意，但现在说何时正式发布还为时尚早。我们很高兴能把这一新版本推向社区，它们会告诉我们应该如何开发这一软件。”
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