如下两种执行计划，11G的那个到底优越在那里？
物理I/O减少了？怎么减少的呢？谢谢
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;
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, 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.
11.3.3.1.2 New Implementation for Nested Loop Joins
Oracle Database 11g 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. Oracle Database 11g 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, 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. In this case, Oracle Database uses the original implementation for nested loop joins.
|
