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
| Property | Description |
|---|---|
| Parameter type | Integer |
| Default value | 8388608 (8 MB) |
| Modifiable | No |
| Range of values | Operating system-dependent |
| Basic | No |
Note:
Oracle does not recommend using theCREATE_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.
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