More nested loops in 11g : A change in nested loops around in 11g

Original and New Implementation for Nested Loop Joins

Oracle Database 11g Release 1 (11.1) introduces a new implementation for nested loop joins. As a result, execution plans that include nested loops might appear different than they did in previous releases of Oracle Database. Both the new implementation and the original implementation for nested loop joins are possible in Oracle Database 11g Release 1 (11.1). So, when analyzing execution plans, it is important to understand that the number of NESTED LOOPS join row sources might be different.
Original Implementation for Nested Loop Joins

Consider the following query:

SELECT e.first_name, e.last_name, e.salary, d.department_name
    FROM hr.employees e, hr.departments d
    WHERE d.department_name IN ('Marketing', 'Sales')
      AND e.department_id = d.department_id;

before Oracle Database 11g Release 1 (11.1), the execution plan for this query might appear similar to the following execution plan:

-------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                   |    19 |   722 |     3  (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMPLOYEES         |    10 |   220 |     1  (0)| 00:00:01 |
|   2 |   NESTED LOOPS              |                   |    19 |   722 |     3  (0)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL        | DEPARTMENTS       |     2 |    32 |     2  (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN         | EMP_DEPARTMENT_IX |    10 |       |     0  (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - filter("D"."DEPARTMENT_NAME"='Marketing' OR "D"."DEPARTMENT_NAME"='Sales')
   4 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

In this example,

• the outer side of the join consists of a scan of the hr.departments table that returns the rows that match the condition department_name IN ('Marketing', 'Sales').
• The inner loop retrieves the employees in the hr.employees table that are associated with those departments.

New Implementation for Nested Loop Joins
 
Oracle Database 11g Release 1 (11.1) introduces a new implementation for nested loop joins to reduce overall latency for physical I/O. When an index or a table block is not in the buffer cache and is needed to process the join, a physical I/O is required. In Oracle Database 11g Release 1 (11.1), Oracle Database can batch multiple physical I/O requests and process them using a vector I/O instead of processing them one at a time. As part of the new implementation for nested loop joins, two NESTED LOOPS join row sources might appear in the execution plan where only one would have appeared in prior releases. In such cases, Oracle Database allocates one NESTED LOOPS join row source to join the values from the table on the outer side of the join with the index on the inner side. A second row source is allocated to join the result of the first join, which includes the rowids stored in the index, with the table on the inner side of the join.

Consider the query in "Original Implementation for Nested Loop Joins". In Oracle Database 11g Release 1 (11.1), with the new implementation for nested loop joins, the execution plan for this query might appear similar to the following execution plan:

------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name              | Rows  | Bytes | Cost(%CPU)| Time      |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                   |    19 |   722 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |                   |       |       |            |          |
|   2 |   NESTED LOOPS               |                   |    19 |   722 |     3   (0)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL         | DEPARTMENTS       |     2 |    32 |     2   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | EMP_DEPARTMENT_IX |    10 |       |     0   (0)| 00:00:01 |
|   5 |   TABLE ACCESS BY INDEX ROWID| EMPLOYEES         |    10 |   220 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - filter("D"."DEPARTMENT_NAME"='Marketing' OR "D"."DEPARTMENT_NAME"='Sales')
   4 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

In this case, the rows from the hr.departments table constitute the outer side of the first join.

• The inner side of the first join is the index emp_department_ix.
• The results of the first join constitute the outer side of the second join, which has the hr.employees table as its inner side.

There are cases where a second join row source is not allocated, and the execution plan looks the same as it did in prior releases. The following list describes such cases:

• All of the columns needed from the inner side of the join are present in the index, and there is no table access required. In this case, Oracle Database allocates only one join row source.
• The order of the rows returned might be different than it was in previous releases.
• Hence, when Oracle Database tries to preserve a specific ordering of the rows, for example to eliminate the need for an ORDER BY sort, Oracle Database might use the original implementation for nested loop joins.
• The OPTIMIZER_FEATURES_ENABLE initialization parameter is set to a release before Oracle Database 11g Release 1 (11.1). In this case, Oracle Database uses the original implementation for nested loop joins.

-Thanks

Geek DBA

Source: Oracle 11g Documentation