COLCARDF (see number 4 in Figure 5) : column cardinality, which denotes the estimated number of distinct values in the column.
COLCARDF(见图5中4)：列基数，表示评估的列中不同值的数目。
To achieve this goal, the partitioning key should be selected on the columns that have high cardinality, or in other words, that contain a large number of distinct values.
为了实现此目标，分区键应该在具有较高基数(换言之，即包含大量不同值)的列上选择。
The indexed columns have low cardinality, that is, the number of distinct values is small compared to the number of table rows.
索引列有较低的相似度，这就意味着，在表的记录中相同值的数据的数量比较少。
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