cardinality(基数，集的势）：表示一个行集的行数。
1. Base cardinality is the number of rows in a base table.
2. Effective cardinality is the number of rows that are selected from a base table.
3. Join cardinality is the number of rows produced when two row sets are joined together.
4. Distinct cardinality is the number of distinct values in a column of a row set.
5. Group cardinality is the number of rows produced from a row set after the GROUP BY operator is applied.
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