Cardinality指什麼？

warehouse發表於2008-04-25

在效能最佳化嚮導中經常會出現Cardinality，Cardinality到底是指什麼，看看doc的準確解釋：

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

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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.

來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/19602/viewspace-1002989/，如需轉載，請註明出處，否則將追究法律責任。

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