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Synopsis

CARDINALITY (object_name, number) A hint to specify the expected record set size in a specific operation. OBJECT_NAME can be:
  1. A specific table. Contains the expected number of rows in the table. For example:
SELECT /*++ CARDINALITY (o, 100) */ s.name, o.id
FROM staff s JOIN org o
ON s.deptno = o.deptno
WHERE o.division = 'EASTERN';
Here the hint indicates that there are 100 rows in the org table (for the purpose of query optimization). It doesn’t say anything about how many of them have division = ’EASTERN’.
  1. An alias given to a sub-query result. Contains the expected results from it as if it was done without push down/up. The sub-query must be aliased, and the alias name passed as the argument of the hint.
For example: 
SELECT /*++ CARDINALITY (east_deptnumb, 10000) */
s.name, east_deptnumb.id
FROM staff s JOIN (SELECT o.deptno ,id
                   FROM org o
                   WHERE o.division = 'EASTERN') AS east_deptnumb
ON s.deptno = east_deptnumb.deptno;
Specifying multiple hints with the same object_name and different numbers is contradictory and is not allowed.