（8）连接基数
1、基本的连接基数

oracle有两个公式用于连接基数的计算：

SQL> set autotrace traceonly explain;SQL> select t1.v1, t2.v1  2    from t1, t2  3   where t1.filter = 1  4     and t2.join1 = t1.join1  5     and t2.filter = 1;执行计划----------------------Plan hash value: 2959412835---------------------------------------| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------|   0 | SELECT STATEMENT   |      |  1782 | 60588 |    67   (2)| 00:00:01 ||*  1 |  HASH JOIN         |      |  1782 | 60588 |    67   (2)| 00:00:01 ||*  2 |   TABLE ACCESS FULL| T2   |   198 |  3366 |    33   (0)| 00:00:01 ||*  3 |   TABLE ACCESS FULL| T1   |   392 |  6664 |    33   (0)| 00:00:01 |---------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - access("T2"."JOIN1"="T1"."JOIN1")   2 - filter("T2"."FILTER"=1)   3 - filter("T1"."FILTER"=1)