What is the meaning of this statement.

Of http://docs.oracle.com/cd/E11882_01/server.112/e16638/optimops.htm#autoId34, there is not that I can't understand.
If the path of the inner table is independent of the external table, then the same lines are retrieved for each iteration of the outer loop, significantly reduce the performance of the.
What is the meaning of this statement? You can take an example for me?

Thanks
Lonion

>
Of http://docs.oracle.com/cd/E11882_01/server.112/e16638/optimops.htm#autoId34, there is not that I can't understand.

If the inner table's access path is independent of the outer table, then the same rows are retrieved for every iteration of the outer loop, degrading performance considerably.

What is the meaning of this statement? You can take an example for me?
>
Can you say: join Cartesian?

This quote is from the section explaining the nested loops. and note it gives you a clue:
>
See also:

"Cartesian joins.
>
The sentence BEFORE the one you quoted, it is what connects your quote with the MENTION:
>
It is important to ensure that the internal table is driven out of the external table (function).
>
This statement means that the lines of the internal table should DEPEND ON the external table.

In a Cartesian join the inner table will depend on the external table at all:

SELECT D.*, E.* FROM DEPT D, EMP E

There is no WHERE clause, so there is nothing saying Oracle tables are related as well. Oracle will perform a Cartesian join and if a nested loop is used then, as says your quote, "the same lines are retrieved for each iteration of the outer loop, performance degradation significantly."

All Oracle ranks visits to query external table lines will now be the inner table. But because there is no WHERE clause is no available information to EXCLUDE lines from the internal table "the same lines are extracted" (ALL) "for each iteration of the outer loop.

Here is the same query above using the USE_NL hint to force Oracle to use a nested loop

SQL> select /*+ use_nl (d e) */ d.*, e.* from dept d, emp e;

56 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 4192419542

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    56 |  3248 |    10   (0)| 00:00:01 |
|   1 |  NESTED LOOPS      |      |    56 |  3248 |    10   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    80 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |    14 |   532 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         42  consistent gets
          0  physical reads
          0  redo size
       3897  bytes sent via SQL*Net to client
        452  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         56  rows processed

SQL>

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