ITPub博客

首页 > Linux操作系统 > Linux操作系统 > lob字段的retention与pctversion参数(zt)

lob字段的retention与pctversion参数(zt)

原创 Linux操作系统 作者:tolywang 时间:2006-08-10 00:00:00 0 删除 编辑

http://xzh2000.itpub.net/post/96/77393

Lob字段默认是pctversion等到于10,意思是表lob字段所在的表空间需要预留10%的空间给lob的前映象使用,当表空间比较小时,用pctversion应该不会有大的问题,当表空间超过100g时,就需要预留10g,可能存在浪费空间的问题,比如每小时更新lob记录的频率增长不多,还要减去(当lob的内容小于4000字节时,lob的前映象保存在undo表空间)这部分,但如果该表空间增长到300g,就有30g的空间ち粝吕幢赣?导致较多的空间被浪费.所以,当您的LOB字段所在的表空间已经很大时,要注意pctversion参数要适当的调小,否则空间的浪费是相当的严重...


关于lob的retention参数,这个与undo表空间的undo_retention是一致的,如果你的lob更新较小,就可以通过修改undo_retention来调整lob前映象的失效时间,由于lob的retention参数是与undo_retention保持一致,所以修改undo_retention来说,一定要慎重,毕竟它将会影响库中所有lob字段的retention参数...

UNDO_RETENTION specifies (in seconds) the low threshold value of undo retention. For AUTOEXTEND undo tablespaces, the system retains undo for at least the time specified in this parameter, and automatically tunes the undo retention period to satisfy the undo requirements of the queries. For fixed- size undo tablespaces, the system automatically tunes for the maximum possible undo retention period, based on undo tablespace size and usage history, and ignores UNDO_RETENTION unless retention guarantee is enabled.

The setting of this parameter should account for any flashback requirements of the system. Automatic tuning of undo retention is not supported for LOBs. The RETENTION value for LOB columns is set to the value of the UNDO_RETENTION parameter.

The UNDO_RETENTION parameter can only be honored if the current undo tablespace has enough space. If an active transaction requires undo space and the undo tablespace does not have available space, then the system starts reusing unexpired undo space. This action can potentially cause some queries to fail with a "snapshot too old" message.

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/35489/viewspace-84426/,如需转载,请注明出处,否则将追究法律责任。

请登录后发表评论 登录
全部评论
Oracle , MySQL, SAP IQ, SAP HANA, PostgreSQL, Tableau 技术讨论,希望在这里一起分享知识,讨论技术,畅谈人生 。

注册时间:2007-12-10

  • 博文量
    5595
  • 访问量
    13102860