ITPub博客

首页 > 应用开发 > IT综合 > Cardinality指什么?

Cardinality指什么?

原创 IT综合 作者:warehouse 时间:2008-04-25 22:56:18 0 删除 编辑

在性能优化向导中经常会出现Cardinality,Cardinality到底是指什么,看看doc的准确解释:

Cardinality is the number of unique values in a column in relation to the number of rows in the table

[@more@]

CREATE_BITMAP_AREA_SIZE

PropertyDescription
Parameter typeInteger
Default value8388608 (8 MB)
ModifiableNo
Range of valuesOperating system-dependent
BasicNo

Note:

Oracle does not recommend using the CREATE_BITMAP_AREA_SIZE parameter unless the instance is configured with the shared server option. Oracle recommends that you enable automatic sizing of SQL working areas by setting PGA_AGGREGATE_TARGET instead. CREATE_BITMAP_AREA_SIZE is retained for backward compatibility.

CREATE_BITMAP_AREA_SIZE is relevant only for systems containing bitmap indexes. It specifies the amount of memory (in bytes) allocated for bitmap creation. The default value is 8 MB. A larger value may speed up index creation.

Cardinality is the number of unique values in a column in relation to the number of rows in the table. If cardinality is very small, you can set a small value for this parameter. For example, if cardinality is only 2, then the value can be on the order of kilobytes rather than megabytes. As a general rule, the higher the cardinality, the more memory is needed for optimal performance.

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/19602/viewspace-1002989/,如需转载,请注明出处,否则将追究法律责任。

下一篇: Optimizer Hints!
请登录后发表评论 登录
全部评论

注册时间:2007-12-07

  • 博文量
    717
  • 访问量
    5114458