The truth about the IMBRIQUEE LOOP

Hi Experts,

Yesterday, I looking for the differences between IN and there ARE operators. I googled and I read a thread in the site of Tom Kyte (http://asktom.oracle.com/pls/asktom/f?p=100:11:P11_QUESTION_ID:953229842074). In short, Tom wrote that the use of these two operators are different. They are also treated differently by the optimizer. IN operator works more like join two tables (hased, sorting, etc.) On the other hand, works longer EXIST as LOOP IMBRIQUEE. In other words, the inner loop runs once for each row in the outer loop. He also suggested that if the sub query is very large and high query in small. And let's say that the index of use of subquery, operator EXIST becomes much more appropriate. However, if the subquery is small FOR operator to become effective.

In addition, Jonathan Lewis also mentions the nested loops in his book (based on Oracle Fundemantals costs) with a chapter. He says the following.

"The execution plan of a join on the inner table index nested loops can have two different forms of 9i from: a when the optimizer uses the index on the inner table for a single scan and another when the optimizer uses the index for a range scan." The second form ceases to be an option, however, if the external table is guaranteed to return a single line. »

My question is, if you look at the loop nested within query plans. It always works as a line returns unique index for Each row of the outer query as follows. I mean that you can come through a loop nested between two tables with FULL TABLE SCAN (usually used for this hash join). The optimizer work like this? If the join condition contains indexes between two tables and one of them is quite huge according to the other. Then the optimizer always uses NESTED LOOP rather than other join methods?

If the above is correct, can we write the following formula?

NESTED LOOP = QUERY EXTERNAL (a low num lines FTS) & SUB QUERY (a high number of lines with INDEX)

----------------------------------------------------------------------------------------

| ID | Operation | Name | Lines | Bytes | Cost (% CPU). Time |

----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT |         |     1.    58.     4 (0) | 00:00:01 |

|   1.  NESTED LOOPS |         |       |       |            |          |

|   2.   NESTED LOOPS |         |     1.    58.     4 (0) | 00:00:01 |

|*  3 |    TABLE ACCESS FULL | EMP |     1.    38.     3 (0) | 00:00:01 |

|*  4 |    INDEX UNIQUE SCAN | PK_DEPT |     1.       |     0 (0) | 00:00:01 |

|   5.   TABLE ACCESS BY INDEX ROWID| DEPT |     1.    20.     1 (0) | 00:00:01 |

----------------------------------------------------------------------------------------

Concerning

Charlie

The optimizer uses nested loops"when the following conditions are met:

  • The database attached to small subsets of data.
  • The join condition is an effective way to access the second table. ":

The query optimizer

Tags: Database

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