| Oracle® Database Performance Tuning Guide 10g Release 1 (10.1) Part Number B10752-01 |
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| Oracle® Database Performance Tuning Guide 10g Release 1 (10.1) Part Number B10752-01 |
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View PDF |
This chapter introduces execution plans, describes the SQL statement EXPLAIN PLAN, and explains how to interpret its output. This chapter also provides procedures for managing outlines to control application performance characteristics.
This chapter contains the following sections:
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The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's execution plan is the sequence of operations Oracle performs to run the statement.
The row source tree is the core of the execution plan. It shows the following information:
In addition to the row source tree, the plan table contains information about the following:
The EXPLAIN PLAN results let you determine whether the optimizer selects a particular execution plan, such as, nested loops join. It also helps you to understand the optimizer decisions, such as why the optimizer chose a nested loops join instead of a hash join, and lets you understand the performance of a query.
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Note: Oracle Performance Manager charts and Oracle SQL Analyze can automatically create and display explain plans for you. For more information on using explain plans, see Oracle Enterprise Manager Concepts. |
With the query optimizer, execution plans can and do change as the underlying optimizer inputs change. EXPLAIN PLAN output shows how Oracle runs the SQL statement when the statement was explained. This can differ from the plan during actual execution for a SQL statement, because of differences in the execution environment and explain plan environment.
Execution plans can differ due to the following:
Even if the schemas are the same, the optimizer can choose different execution plans if the costs are different. Some factors that affect the costs include the following:
Examining an explain plan lets you look for throw-away in cases such as the following:
For example, in the following explain plan, the last step is a very unselective range scan that is executed 76563 times, accesses 11432983 rows, throws away 99% of them, and retains 76563 rows. Why access 11432983 rows to realize that only 76563 rows are needed?
Rows Execution Plan -------- ---------------------------------------------------- 12 SORT AGGREGATE 2 SORT GROUP BY 76563 NESTED LOOPS 76575 NESTED LOOPS 19 TABLE ACCESS FULL CN_PAYRUNS_ALL 76570 TABLE ACCESS BY INDEX ROWID CN_POSTING_DETAILS_ALL 76570 INDEX RANGE SCAN (object id 178321) 76563 TABLE ACCESS BY INDEX ROWID CN_PAYMENT_WORKSHEETS_ALL 11432983 INDEX RANGE SCAN (object id 186024)
The execution plan operation alone cannot differentiate between well-tuned statements and those that perform poorly. For example, an EXPLAIN PLAN output that shows that a statement uses an index does not necessarily mean that the statement runs efficiently. Sometimes indexes can be extremely inefficient. In this case, you should examine the following:
It is best to use EXPLAIN PLAN to determine an access plan, and then later prove that it is the optimal plan through testing. When evaluating a plan, examine the statement's actual resource consumption.
In addition to running the EXPLAIN PLAN command and displaying the plan, you can use the V$SQL_PLAN views to display the execution plan of a SQL statement:
After the statement has executed, you can display the plan by querying the V$SQL_PLAN view. V$SQL_PLAN contains the execution plan for every statement stored in the cursor cache. Its definition is similar to the PLAN_TABLE. See "PLAN_TABLE Columns".
The advantage of V$SQL_PLAN over EXPLAIN PLAN is that you do not need to know the compilation environment that was used to execute a particular statement. For EXPLAIN PLAN, you would need to set up an identical environment to get the same plan when executing the statement.
The V$SQL_PLAN_STATISTICS view provides the actual execution statistics for every operation in the plan, such as the number of output rows and elapsed time. All statistics, except the number of output rows, are cumulative. For example, the statistics for a join operation also includes the statistics for its two inputs. The statistics in V$SQL_PLAN_STATISTICS are available for cursors that have been compiled with the STATISTICS_LEVEL initialization parameter set to ALL.
The V$SQL_PLAN_STATISTICS_ALL view enables side by side comparisons of the estimates that the optimizer provides for the number of rows and elapsed time. This view combines both V$SQL_PLAN and V$SQL_PLAN_STATISTICS information for every cursor.
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Oracle does not support EXPLAIN PLAN for statements performing implicit type conversion of date bind variables. With bind variables in general, the EXPLAIN PLAN output might not represent the real execution plan.
From the text of a SQL statement, TKPROF cannot determine the types of the bind variables. It assumes that the type is CHARACTER, and gives an error message if this is not the case. You can avoid this limitation by putting appropriate type conversions in the SQL statement.
The PLAN_TABLE is automatically created as a global temporary table to hold the output of an EXPLAIN PLAN statement for all users. PLAN_TABLE is the default sample output table into which the EXPLAIN PLAN statement inserts rows describing execution plans. See "PLAN_TABLE Columns" for a description of the columns in the table.
While a PLAN_TABLE table is automatically set up for each user, you can use the SQL script utlxplan.sql to manually create a local PLAN_TABLE in your schema. The exact name and location of this script depends on your operating system. On Unix, it is located in the $ORACLE_HOME/rdbms/admin directory.
For example, run the commands in Example 19-2 from a SQL*Plus session to create the PLAN_TABLE in the HR schema.
CONNECT HR/your_password @$ORACLE_HOME/rdbms/admin/utlxplan.sql Table created.
Oracle Corporation recommends that you drop and rebuild your local PLAN_TABLE table after upgrading the version of the database because the columns might change. This can cause scripts to fail or cause TKPROF to fail, if you are specifying the table.
If you want an output table with a different name, first create PLAN_TABLE manually with the utlxplan.sql script and then rename the table with the RENAME SQL statement. For example:
RENAME PLAN_TABLE TO my_plan_table;
To explain a SQL statement, use the EXPLAIN PLAN FOR clause immediately before the statement. For example:
EXPLAIN PLAN FOR SELECT last_name FROM employees;
This explains the plan into the PLAN_TABLE table. You can then select the execution plan from PLAN_TABLE. See "Displaying PLAN_TABLE Output".
With multiple statements, you can specify a statement identifier and use that to identify your specific execution plan. Before using SET STATEMENT ID, remove any existing rows for that statement ID.
In Example 19-3, st1 is specified as the statement identifier:
EXPLAIN PLAN SET STATEMENT_ID = 'st1' FOR SELECT last_name FROM employees;
You can specify the INTO clause to specify a different table.
EXPLAIN PLAN INTO my_plan_table FOR SELECT last_name FROM employees;
You can specify a statement Id when using the INTO clause.
EXPLAIN PLAN SET STATEMENT_ID = 'st1' INTO my_plan_table FOR SELECT last_name FROM employees;
| See Also:
Oracle Database SQL Reference for a complete description of |
After you have explained the plan, use the following SQL scripts or PL/SQL package provided by Oracle to display the most recent plan table output:
UTLXPLS.SQL
This script displays the plan table output for serial processing. Example 14-2, "EXPLAIN PLAN Output" is an example of the plan table output when using the UTLXPLS.SQL script.
UTLXPLP.SQL
This script displays the plan table output including parallel execution columns.
DBMS_XPLAN.DISPLAY procedure
This procedure accepts options for displaying the plan table output. You can specify:
PLAN_TABLEEXPLAIN PLANBASIC, SERIAL, and TYPICAL, ALL,Some examples of the use of DBMS_XPLAN to display PLAN_TABLE output are:
SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY()); SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY('MY_PLAN_TABLE', 'st1','TYPICAL'));
| See Also:
PL/SQL Packages and Types Reference for more information on the |
If you have specified a statement identifier, then you can write your own script to query the PLAN_TABLE. For example:
STATEMENT_ID.CONNECT BY clause to walk the tree from parent to child, the join keys being STATEMENT_ID = PRIOR STATEMENT_ID and PARENT_ID = PRIOR ID.LEVEL (associated with CONNECT BY) to indent the children.
SELECT cardinality "Rows", lpad(' ',level-1)||operation||' '||options||' '||object_name "Plan" FROM PLAN_TABLE CONNECT BY prior id = parent_id AND prior statement_id = statement_id START WITH id = 0 AND statement_id = 'st1' ORDER BY id; Rows Plan ------- ---------------------------------------- SELECT STATEMENT TABLE ACCESS FULL EMPLOYEES
The NULL in the Rows column indicates that the optimizer does not have any statistics on the table. Analyzing the table shows the following:
Rows Plan ------- ---------------------------------------- 16957 SELECT STATEMENT 16957 TABLE ACCESS FULL EMPLOYEES
You can also select the COST. This is useful for comparing execution plans or for understanding why the optimizer chooses one execution plan over another.
This section uses EXPLAIN PLAN examples to illustrate execution plans. The statement in Example 19-5 is used to display the execution plans.
SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'statement_id','BASIC'));
Examples of the output from this statement are shown in Example 19-6 and Example 19-7.
EXPLAIN PLAN SET statement_id = 'ex_plan1' FOR SELECT phone_number FROM employees WHERE phone_number LIKE '650%'; --------------------------------------- | Id | Operation | Name | --------------------------------------- | 0 | SELECT STATEMENT | | | 1 | TABLE ACCESS FULL| EMPLOYEES | ---------------------------------------
This plan shows execution of a SELECT statement. The table employees is accessed using a full table scan.
employees is accessed, and the WHERE clause criteria is evaluated for every row.SELECT statement returns the rows meeting the WHERE clause criteria.EXPLAIN PLAN SET statement_id = 'ex_plan2' FOR SELECT last_name FROM employees WHERE last_name LIKE 'Pe%'; SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'ex_plan2','BASIC')); ---------------------------------------- | Id | Operation | Name | ---------------------------------------- | 0 | SELECT STATEMENT | | | 1 | INDEX RANGE SCAN| EMP_NAME_IX | ----------------------------------------
This plan shows execution of a SELECT statement.
EMP_NAME_IX is used in a range scan operation to evaluate the WHERE clause criteria.SELECT statement returns rows satisfying the WHERE clause conditions.Tuning a parallel query begins much like a non-parallel query tuning exercise by choosing the driving table. However, the rules governing the choice are different. In the non-parallel case, the best driving table is typically the one that produces fewest number of rows after limiting conditions are applied. The small number of rows are joined to larger tables using non-unique indexes. For example, consider a table hierarchy consisting of CUSTOMER, ACCOUNT, and TRANSACTION.
Text description of the illustration pfgrf211.gif
CUSTOMER is the smallest table while TRANSACTION is the largest. A typical OLTP query might be to retrieve transaction information about a particular customer's account. The query would drive from the CUSTOMER table. The goal in this case is to minimize logical I/O, which typically minimizes other critical resources including physical I/O and CPU time.
For parallel queries, the choice of the driving table is usually the largest table because parallel query can be utilized. Obviously, it would not be efficient to use parallel query on the query, because only a few rows from each table are ultimately accessed. However, what if it were necessary to identify all customers that had transactions of a certain type last month? It would be more efficient to drive from the TRANSACTION table because there are no limiting conditions on the customer table. The rows from the TRANSACTION table would be joined to the ACCOUNT table, and finally to the CUSTOMER table. In this case, the indexes utilized on the ACCOUNT and CUSTOMER table are likely to be highly selective primary key or unique indexes, rather than non-unique indexes used in the first query. Because the TRANSACTION table is large and the column is un-selective, it would be beneficial to utilize parallel query driving from the TRANSACTION table.
Parallel operations include:
PARALLEL_TO_PARALLELPARALLEL_TO_SERIAL
A PARALLEL_TO_SERIAL operation which is always the step that occurs when rows from a parallel operation are consumed by the query coordinator. Another type of operation that does not occur in this query is a SERIAL operation. If these types of operations occur, consider making them parallel operations to improve performance because they too are potential bottlenecks.
PARALLEL_FROM_SERIALPARALLEL_TO_PARALLEL
PARALLEL_TO_PARALLEL operations generally produce the best performance as long as the workloads in each step are relatively equivalent.
PARALLEL_COMBINED_WITH_CHILDPARALLEL_COMBINED_WITH_PARENT
A PARALLEL_COMBINED_WITH_PARENT operation occurs when the step is performed simultaneously with the parent step.
If a parallel step produces many rows, the query coordinator (QC) may not be able to consume them as fast as they are being produced. There is little that can be done to improve this.
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When using EXPLAIN PLAN with parallel queries, one parallel plan is compiled and executed. This plan is derived from the serial plan by allocating row sources specific to the parallel support in the Query Coordinator (QC) plan. The table queue row sources (PX Send and PX Receive), the granule iterator, and buffer sorts, required by the two slave set PQ model, are directly inserted into the parallel plan. This plan is the exact same plan for all the slaves if executed in parallel or for the QC if executed in serial.
Example 19-8 is a simple query for illustrating an EXPLAIN PLAN for a parallel query.
CREATE TABLE emp2 AS SELECT * FROM employees; ALTER TABLE emp2 PARALLEL 2; EXPLAIN PLAN FOR SELECT SUM(salary) FROM emp2 GROUP BY department_id; SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
-------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | TQ |IN-OUT| PQ Distrib | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 41 | 1066 | 4 | | | | | 1 | PX COORDINATOR | | | | | | | | | 2 | PX SEND QC (RANDOM) | :TQ10001 | 41 | 1066 | 4 | Q1,01 | P->S | QC (RAND) | | 3 | SORT GROUP BY | | 41 | 1066 | 4 | Q1,01 | PCWP | | | 4 | PX RECEIVE | | 41 | 1066 | 4 | Q1,01 | PCWP | | | 5 | PX SEND HASH | :TQ10000 | 41 | 1066 | 4 | Q1,00 | P->P | HASH | | 6 | SORT GROUP BY | | 41 | 1066 | 4 | Q1,00 | PCWP | | | 7 | PX BLOCK ITERATOR | | 41 | 1066 | 1 | Q1,00 | PCWC | | | 8 | TABLE ACCESS FULL| EMP2 | 41 | 1066 | 1 | Q1,00 | PCWP | | --------------------------------------------------------------------------------------------------
The table EMP2 is scanned in parallel by one set of slaves while the aggregation for the GROUP BY is done by the second set. The PX BLOCK ITERATOR row source represents the splitting up of the table EMP2 into pieces so as to divide the scan workload between the parallel scan slaves. The PX SEND and PX RECEIVE row sources represent the pipe that connects the two slave sets as rows flow up from the parallel scan, get repartitioned through the HASH table queue, and then read by and aggregated on the top slave set. The PX SEND QC row source represents the aggregated values being sent to the QC (Query Coordinator) in random (RAND) order. The PX COORDINATOR row source represents the QC or Query Coordinator which controls and schedules the parallel plan appearing below it in the plan tree.
Index row sources using bitmap indexes appear in the EXPLAIN PLAN output with the word BITMAP indicating the type of the index. Consider the sample query and plan in Example 19-9.
EXPLAIN PLAN FOR SELECT * FROM t WHERE c1 = 2 AND c2 <> 6 OR c3 BETWEEN 10 AND 20;SELECT STATEMENT TABLE ACCESS T BY INDEX ROWID BITMAP CONVERSION TO ROWID BITMAP OR BITMAP MINUS BITMAP MINUS BITMAP INDEX C1_IND SINGLE VALUE BITMAP INDEX C2_IND SINGLE VALUE BITMAP INDEX C2_IND SINGLE VALUE BITMAP MERGE BITMAP INDEX C3_IND RANGE SCAN
In this example, the predicate c1=2 yields a bitmap from which a subtraction can take place. From this bitmap, the bits in the bitmap for c2 = 6 are subtracted. Also, the bits in the bitmap for c2 IS NULL are subtracted, explaining why there are two MINUS row sources in the plan. The NULL subtraction is necessary for semantic correctness unless the column has a NOT NULL constraint. The TO ROWIDS option is used to generate the ROWIDs that are necessary for the table access.
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Note: Queries using bitmap join index indicate the bitmap join index access path. The operation for bitmap join index is the same as bitmap index. |
Use EXPLAIN PLAN to see how Oracle accesses partitioned objects for specific queries.
Partitions accessed after pruning are shown in the PARTITION START and PARTITION STOP columns. The row source name for the range partition is PARTITION RANGE. For hash partitions, the row source name is PARTITION HASH.
A join is implemented using partial partition-wise join if the DISTRIBUTION column of the plan table of one of the joined tables contains PARTITION(KEY). Partial partition-wise join is possible if one of the joined tables is partitioned on its join column and the table is parallelized.
A join is implemented using full partition-wise join if the partition row source appears before the join row source in the EXPLAIN PLAN output. Full partition-wise joins are possible only if both joined tables are equi-partitioned on their respective join columns. Examples of execution plans for several types of partitioning follow.
Consider the following table, emp_range, partitioned by range on hire_date to illustrate how pruning is displayed. Assume that the tables employees and departments from the Oracle sample schema exist.
CREATE TABLE emp_range PARTITION BY RANGE(hire_date) ( PARTITION emp_p1 VALUES LESS THAN (TO_DATE('1-JAN-1992','DD-MON-YYYY')), PARTITION emp_p2 VALUES LESS THAN (TO_DATE('1-JAN-1994','DD-MON-YYYY')), PARTITION emp_p3 VALUES LESS THAN (TO_DATE('1-JAN-1996','DD-MON-YYYY')), PARTITION emp_p4 VALUES LESS THAN (TO_DATE('1-JAN-1998','DD-MON-YYYY')), PARTITION emp_p5 VALUES LESS THAN (TO_DATE('1-JAN-2001','DD-MON-YYYY')) ) AS SELECT * FROM employees;
For the first example, consider the following statement:
EXPLAIN PLAN FOR SELECT * FROM emp_range;
Oracle displays something similar to the following:
--------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 105 | 13965 | 2 | | | | 1 | PARTITION RANGE ALL| | 105 | 13965 | 2 | 1 | 5 | | 2 | TABLE ACCESS FULL | EMP_RANGE | 105 | 13965 | 2 | 1 | 5 | ---------------------------------------------------------------------------------
A partition row source is created on top of the table access row source. It iterates over the set of partitions to be accessed. In this example, the partition iterator covers all partitions (option ALL), because a predicate was not used for pruning. The PARTITION_START and PARTITION_STOP columns of the PLAN_TABLE show access to all partitions from 1 to 5.
For the next example, consider the following statement:
EXPLAIN PLAN FOR SELECT * FROM emp_range WHERE hire_date >= TO_DATE('1-JAN-1996','DD-MON-YYYY');
-------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 399 | 2 | | | | 1 | PARTITION RANGE ITERATOR| | 3 | 399 | 2 | 4 | 5 | |* 2 | TABLE ACCESS FULL | EMP_RANGE | 3 | 399 | 2 | 4 | 5 | --------------------------------------------------------------------------------------
In the previous example, the partition row source iterates from partition 4 to 5, because we prune the other partitions using a predicate on hire_date.
Finally, consider the following statement:
EXPLAIN PLAN FOR SELECT * FROM emp_range WHERE hire_date < TO_DATE('1-JAN-1992','DD-MON-YYYY');
------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 133 | 2 | | | | 1 | PARTITION RANGE SINGLE| | 1 | 133 | 2 | 1 | 1 | |* 2 | TABLE ACCESS FULL | EMP_RANGE | 1 | 133 | 2 | 1 | 1 | ------------------------------------------------------------------------------------
In the previous example, only partition 1 is accessed and known at compile time; thus, there is no need for a partition row source.
Oracle displays the same information for hash partitioned objects, except the partition row source name is PARTITION HASH instead of PARTITION RANGE. Also, with hash partitioning, pruning is only possible using equality or IN-list predicates.
To illustrate how Oracle displays pruning information for composite partitioned objects, consider the table emp_comp that is range partitioned on hiredate and subpartitioned by hash on deptno.
CREATE TABLE emp_comp PARTITION BY RANGE(hire_date) SUBPARTITION BY HASH(department_id) SUBPARTITIONS 3 ( PARTITION emp_p1 VALUES LESS THAN (TO_DATE('1-JAN-1992','DD-MON-YYYY')), PARTITION emp_p2 VALUES LESS THAN (TO_DATE('1-JAN-1994','DD-MON-YYYY')), PARTITION emp_p3 VALUES LESS THAN (TO_DATE('1-JAN-1996','DD-MON-YYYY')), PARTITION emp_p4 VALUES LESS THAN (TO_DATE('1-JAN-1998','DD-MON-YYYY')), PARTITION emp_p5 VALUES LESS THAN (TO_DATE('1-JAN-2001','DD-MON-YYYY')) ) AS SELECT * FROM employees;
For the first example, consider the following statement:
EXPLAIN PLAN FOR SELECT * FROM emp_comp;
-------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 10120 | 1314K| 78 | | | | 1 | PARTITION RANGE ALL| | 10120 | 1314K| 78 | 1 | 5 | | 2 | PARTITION HASH ALL| | 10120 | 1314K| 78 | 1 | 3 | | 3 | TABLE ACCESS FULL| EMP_COMP | 10120 | 1314K| 78 | 1 | 15 | --------------------------------------------------------------------------------
This example shows the plan when Oracle accesses all subpartitions of all partitions of a composite object. Two partition row sources are used for that purpose: a range partition row source to iterate over the partitions and a hash partition row source to iterate over the subpartitions of each accessed partition.
In the following example, the range partition row source iterates from partition 1 to 5, because no pruning is performed. Within each partition, the hash partition row source iterates over subpartitions 1 to 3 of the current partition. As a result, the table access row source accesses subpartitions 1 to 15. In other words, it accesses all subpartitions of the composite object.
EXPLAIN PLAN FOR SELECT * FROM emp_comp WHERE hire_date = TO_DATE('15-FEB-1998', 'DD-MON-YYYY');
----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 20 | 2660 | 17 | | | | 1 | PARTITION RANGE SINGLE| | 20 | 2660 | 17 | 5 | 5 | | 2 | PARTITION HASH ALL | | 20 | 2660 | 17 | 1 | 3 | |* 3 | TABLE ACCESS FULL | EMP_COMP | 20 | 2660 | 17 | 13 | 15 | -----------------------------------------------------------------------------------
In the previous example, only the last partition, partition 5, is accessed. This partition is known at compile time, so we do not need to show it in the plan. The hash partition row source shows accessing of all subpartitions within that partition; that is, subpartitions 1 to 3, which translates into subpartitions 13 to 15 of the emp_comp table.
Now consider the following statement:
EXPLAIN PLAN FOR SELECT * FROM emp_comp WHERE department_id = 20;
----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 101 | 13433 | 78 | | | | 1 | PARTITION RANGE ALL | | 101 | 13433 | 78 | 1 | 5 | | 2 | PARTITION HASH SINGLE| | 101 | 13433 | 78 | 3 | 3 | |* 3 | TABLE ACCESS FULL | EMP_COMP | 101 | 13433 | 78 | | | -----------------------------------------------------------------------------------
In the previous example, the predicate deptno = 20 enables pruning on the hash dimension within each partition, so Oracle only needs to access a single subpartition. The number of that subpartition is known at compile time, so the hash partition row source is not needed.
Finally, consider the following statement:
VARIABLE dno NUMBER; EXPLAIN PLAN FOR SELECT * FROM emp_comp WHERE department_id = :dno;
----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | Pstart| Pstop | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 101 | 13433 | 78 | | | | 1 | PARTITION RANGE ALL | | 101 | 13433 | 78 | 1 | 5 | | 2 | PARTITION HASH SINGLE| | 101 | 13433 | 78 | KEY | KEY | |* 3 | TABLE ACCESS FULL | EMP_COMP | 101 | 13433 | 78 | | | -----------------------------------------------------------------------------------
The last two examples are the same, except that deptno = 20 has been replaced by department_id = :dno. In this last case, the subpartition number is unknown at compile time, and a hash partition row source is allocated. The option is SINGLE for that row source, because Oracle accesses only one subpartition within each partition. The PARTITION_START and PARTITION_STOP is set to KEY. This means that Oracle determines the number of the subpartition at run time.
In the following example, emp_range_did is joined on the partitioning column department_id and is parallelized. This enables use of partial partition-wise join, because the dept2 table is not partitioned. Oracle dynamically partitions the dept2 table before the join.
CREATE TABLE dept2 AS SELECT * FROM departments; ALTER TABLE dept2 PARALLEL 2; CREATE TABLE emp_range_did PARTITION BY RANGE(department_id) (PARTITION emp_p1 VALUES LESS THAN (150), PARTITION emp_p5 VALUES LESS THAN (MAXVALUE) ) AS SELECT * FROM employees; ALTER TABLE emp_range_did PARALLEL 2; EXPLAIN PLAN FOR SELECT /*+ PQ_DISTRIBUTE(d NONE PARTITION) ORDERED */ e.last_name, d.department_name FROM emp_range_did e , dept2 d WHERE e.department_id = d.department_id ;
------------------------------------------------------------------------------------------------------------- | Id| Operation |Name |Rows | Bytes |Cost|Pstart|Pstop| TQ |IN-OUT|PQ Distrib | ------------------------------------------------------------------------------------------------------------- | 0| SELECT STATEMENT | | 284 | 16188 | 6 | | | | | | 1| PX COORDINATOR | | | | | | | | | | | 2| PX SEND QC (RANDOM) |:TQ10001 | 284 | 16188 | 6 | | | Q1,01 | P->S | QC (RAND) | |* 3| HASH JOIN | | 284 | 16188 | 6 | | | Q1,01 | PCWP | | | 4| PX PARTITION RANGE ALL | | 284 | 7668 | 2 | 1 | 2 | Q1,01 | PCWC | | | 5| TABLE ACCESS FULL |EMP_RANGE_DID| 284 | 7668 | 2 | 1 | 2 | Q1,01 | PCWP | | | 6| BUFFER SORT | | | | | | | Q1,01 | PCWC | | | 7| PX RECEIVE | | 21 | 630 | 2 | | | Q1,01 | PCWP | | | 8| PX SEND PARTITION (KEY)|:TQ10000 | 21 | 630 | 2 | | | | S->P |PART (KEY) | | 9| TABLE ACCESS FULL |DEPT2 | 21 | 630 | 2 | | | | | | ------------------------------------------------------------------------------------------------------------
The execution plan shows that the table dept2 is scanned serially and all rows with the same partitioning column value of emp_range_did (department_id) are sent through a PART (KEY), or partition key, table queue to the same slave doing the partial partition-wise join.
In the following example, emp_comp is joined on the partitioning column and is parallelized. This enables use of partial partition-wise join, because the dept2 table is not partitioned. Oracle dynamically partitions the dept2 table before the join.
ALTER TABLE emp_comp PARALLEL 2; EXPLAIN PLAN FOR SELECT /*+ PQ_DISTRIBUTE(d NONE PARTITION) ORDERED */ e.last_name, d.department_name FROM emp_comp e, dept2 d WHERE e.department_id = d.department_id; SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost |Pstart|Pstop| TQ |IN-OUT| PQ Distrib | ------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 445 | 17800 | 5 | | | | | | | 1 | PX COORDINATOR | | | | | | | | | | | 2 | PX SEND QC (RANDOM) |:TQ10001 | 445 | 17800 | 5 | | | Q1,01 | P->S | QC (RAND) | |* 3 | HASH JOIN | | 445 | 17800 | 5 | | | Q1,01 | PCWP | | | 4 | PX PARTITION RANGE ALL | | 107 | 1070 | 3 | 1 | 5 | Q1,01 | PCWC | | | 5 | PX PARTITION HASH ALL | | 107 | 1070 | 3 | 1 | 3 | Q1,01 | PCWC | | | 6 | TABLE ACCESS FULL |EMP_COMP | 107 | 1070 | 3 | 1 | 15 | Q1,01 | PCWP | | | 7 | PX RECEIVE | | 21 | 630 | 1 | | | Q1,01 | PCWP | | | 8 | PX SEND PARTITION (KEY)|:TQ10000 | 21 | 630 | 1 | | | Q1,00 | P->P |PART (KEY) | | 9 | PX BLOCK ITERATOR | | 21 | 630 | 1 | | | Q1,00 | PCWC | | | 10 | TABLE ACCESS FULL |DEPT2 | 21 | 630 | 1 | | | Q1,00 | PCWP | | -------------------------------------------------------------------------------------------------------------
The plan shows that the optimizer selects partial partition-wise join from one of two columns. The PX SEND node type is PARTITION(KEY) and the PQ Distrib column contains the text PART (KEY), or partition key. This implies that the table dept2 is re-partitioned based on the join column department_id to be sent to the parallel slaves executing the scan of EMP_COMP and the join.
Note that in both Example 19-10 and Example 19-11 the PQ_DISTRIBUTE hint is used to explicitly force a partial partition-wise join because the query optimizer could have chosen a different plan based on cost in this query.
In the following example, emp_comp and dept_hash are joined on their hash partitioning columns. This enables use of full partition-wise join. The PARTITION HASH row source appears on top of the join row source in the plan table output.
The PX PARTITION HASH row source appears on top of the join row source in the plan table output while the PX PARTITION RANGE row source appears over the scan of emp_comp. Each parallel slave performs the join of an entire hash partition of emp_comp with an entire partition of dept_hash.
CREATE TABLE dept_hash PARTITION BY HASH(department_id) PARTITIONS 3 PARALLEL 2 AS SELECT * FROM departments; EXPLAIN PLAN FOR SELECT /*+ PQ_DISTRIBUTE(e NONE NONE) ORDERED */ e.last_name, d.department_name FROM emp_comp e, dept_hash d WHERE e.department_id = d.department_id;
------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows |Bytes |Cost |Pstart|Pstop | TQ |IN-OUT| PQ Distrib | ------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 106 | 2544 | 8 | | | | | | | 1 | PX COORDINATOR | | | | | | | | | | | 2 | PX SEND QC (RANDOM) | :TQ10000 | 106 | 2544 | 8 | | | Q1,00 | P->S | QC (RAND) | | 3 | PX PARTITION HASH ALL | | 106 | 2544 | 8 | 1 | 3 | Q1,00 | PCWC | | |* 4 | HASH JOIN | | 106 | 2544 | 8 | | | Q1,00 | PCWP | | | 5 | PX PARTITION RANGE ALL| | 107 | 1070 | 3 | 1 | 5 | Q1,00 | PCWC | | | 6 | TABLE ACCESS FULL | EMP_COMP | 107 | 1070 | 3 | 1 | 15 | Q1,00 | PCWP | | | 7 | TABLE ACCESS FULL | DEPT_HASH | 27 | 378 | 4 | 1 | 3 | Q1,00 | PCWP | | -------------------------------------------------------------------------------------------------------------
An INLIST ITERATOR operation appears in the EXPLAIN PLAN output if an index implements an IN-list predicate. For example:
SELECT * FROM emp WHERE empno IN (7876, 7900, 7902);
The EXPLAIN PLAN output appears as follows:
OPERATION OPTIONS OBJECT_NAME ---------------- --------------- -------------- SELECT STATEMENT INLIST ITERATOR TABLE ACCESS BY ROWID EMP INDEX RANGE SCAN EMP_EMPNO
The INLIST ITERATOR operation iterates over the next operation in the plan for each value in the IN-list predicate. For partitioned tables and indexes, the three possible types of IN-list columns are described in the following sections.
If the IN-list column empno is an index column but not a partition column, then the plan is as follows (the IN-list operator appears before the table operation but after the partition operation):
OPERATION OPTIONS OBJECT_NAME PARTITION_START PARTITION_STOP ---------------- ------------ ----------- --------------- -------------- SELECT STATEMENT PARTITION RANGE ALL KEY(INLIST) KEY(INLIST) INLIST ITERATOR TABLE ACCESS BY LOCAL INDEX ROWID EMP KEY(INLIST) KEY(INLIST) INDEX RANGE SCAN EMP_EMPNO KEY(INLIST) KEY(INLIST)
The KEY(INLIST) designation for the partition start and stop keys specifies that an IN-list predicate appears on the index start/stop keys.
If empno is an indexed and a partition column, then the plan contains an INLIST ITERATOR operation before the partition operation:
OPERATION OPTIONS OBJECT_NAME PARTITION_START PARTITION_STOP ---------------- ------------ ----------- --------------- -------------- SELECT STATEMENT INLIST ITERATOR PARTITION RANGE ITERATOR KEY(INLIST) KEY(INLIST) TABLE ACCESS BY LOCAL INDEX ROWID EMP KEY(INLIST) KEY(INLIST) INDEX RANGE SCAN EMP_EMPNO KEY(INLIST) KEY(INLIST)
If empno is a partition column and there are no indexes, then no INLIST ITERATOR operation is allocated:
OPERATION OPTIONS OBJECT_NAME PARTITION_START PARTITION_STOP ---------------- ------------ ----------- --------------- -------------- SELECT STATEMENT PARTITION RANGE INLIST KEY(INLIST) KEY(INLIST) TABLE ACCESS FULL EMP KEY(INLIST) KEY(INLIST)
If emp_empno is a bitmap index, then the plan is as follows:
OPERATION OPTIONS OBJECT_NAME ---------------- --------------- -------------- SELECT STATEMENT INLIST ITERATOR TABLE ACCESS BY INDEX ROWID EMP BITMAP CONVERSION TO ROWIDS BITMAP INDEX SINGLE VALUE EMP_EMPNO
You can also use EXPLAIN PLAN to derive user-defined CPU and I/O costs for domain indexes. EXPLAIN PLAN displays these statistics in the OTHER column of PLAN_TABLE.
For example, assume table emp has user-defined operator CONTAINS with a domain index emp_resume on the resume column, and the index type of emp_resume supports the operator CONTAINS. Then the query:
SELECT * FROM emp WHERE CONTAINS(resume, 'Oracle') = 1
might display the following plan:
OPERATION OPTIONS OBJECT_NAME OTHER ----------------- ----------- ------------ ---------------- SELECT STATEMENT TABLE ACCESS BY ROWID EMP DOMAIN INDEX EMP_RESUME CPU: 300, I/O: 4
The PLAN_TABLE used by the EXPLAIN PLAN statement contains the columns listed in Table 19-1.
| Column | Type | Description |
|---|---|---|
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Value of the optional |
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Unique identifier of a plan in the database. |
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Date and time when the |
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Any comment (of up to 80 bytes) you want to associate with each step of the explained plan. This column is used to indicate whether an outline or SQL Profile was used for the query. If you need to add or change a remark on any row of the |
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Name of the internal operation performed in this step. In the first row generated for a statement, the column contains one of the following values: See Table 19-3 for more information on values for this column. |
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A variation on the operation described in the See Table 19-3 for more information on values for this column. |
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Name of the database link used to reference the object (a table name or view name). For local queries using parallel execution, this column describes the order in which output from operations is consumed. |
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Name of the user who owns the schema containing the table or index. |
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Name of the table or index. |
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Unique alias of a table or view in a SQL statement. For indexes, it is the object alias of the underlying table. |
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Number corresponding to the ordinal position of the object as it appears in the original statement. The numbering proceeds from left to right, outer to inner with respect to the original statement text. View expansion results in unpredictable numbers. |
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Modifier that provides descriptive information about the object; for example, |
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Current mode of the optimizer. |
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Not currently used. |
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A number assigned to each step in the execution plan. |
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The ID of the next execution step that operates on the output of the |
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Depth of the operation in the row source tree that the plan represents. The value can be used for indenting the rows in a plan table report. |
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For the first row of output, this indicates the optimizer's estimated cost of executing the statement. For the other rows, it indicates the position relative to the other children of the same parent. |
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Cost of the operation as estimated by the optimizer's query approach. Cost is not determined for table access operations. The value of this column does not have any particular unit of measurement; it is merely a weighted value used to compare costs of execution plans. The value of this column is a function of the |
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Estimate by the query optimization approach of the number of rows accessed by the operation. |
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Estimate by the query optimization approach of the number of bytes accessed by the operation. |
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Describes the contents of the
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Start partition of a range of accessed partitions. It can take one of the following values: n indicates that the start partition has been identified by the SQL compiler, and its partition number is given by n.
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Stop partition of a range of accessed partitions. It can take one of the following values: n indicates that the stop partition has been identified by the SQL compiler, and its partition number is given by n.
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Step that has computed the pair of values of the |
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Other information that is specific to the execution step that a user might find useful. See the |
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Method used to distribute rows from producer query servers to consumer query servers. See Table 19-2 for more information on the possible values for this column. For more information about consumer and producer query servers, see Oracle Data Warehousing Guide. |
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CPU cost of the operation as estimated by the query optimizer's approach. The value of this column is proportional to the number of machine cycles required for the operation. For statements that use the rule-based approach, this column is null. |
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I/O cost of the operation as estimated by the query optimizer's approach. The value of this column is proportional to the number of data blocks read by the operation. For statements that use the rule-based approach, this column is null. |
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Temporary space, in bytes, used by the operation as estimated by the query optimizer's approach. For statements that use the rule-based approach, or for operations that do not use any temporary space, this column is null. |
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Predicates used to locate rows in an access structure. For example, start or stop predicates for an index range scan. |
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Predicates used to filter rows before producing them. |
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Expressions produced by the operation. |
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Elapsed time in seconds of the operation as estimated by query optimization. For statements that use the rule-based approach, this column is null. |
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Name of the query block, either system-generated or defined by the user with the |
Table 19-2 describes the values that can appear in the DISTRIBUTION column:
Table 19-3 lists each combination of OPERATION and OPTIONS produced by the EXPLAIN PLAN statement and its meaning within an execution plan.
| Operation | Option | Description |
|---|---|---|
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Operation accepting multiple sets of rowids, returning the intersection of the sets, eliminating duplicates. Used for the single-column indexes access path. |
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Merges several bitmaps resulting from a range scan into one bitmap. |
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Subtracts bits of one bitmap from another. Row source is used for negated predicates. Can be used only if there are nonnegated predicates yielding a bitmap from which the subtraction can take place. An example appears in "Viewing Bitmap Indexes with EXPLAIN PLAN". |
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Computes the bitwise |
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Computes the bitwise |
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Takes each row from a table row source and finds the corresponding bitmap from a bitmap index. This set of bitmaps are then merged into one bitmap in a following |
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|
Retrieves rows in hierarchical order for a query containing a |
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Operation accepting multiple sets of rows returning the union-all of the sets. |
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. |
Operation counting the number of rows selected from a table. |
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Count operation where the number of rows returned is limited by the |
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Retrieval of one or more rowids from a domain index. The options column contain information supplied by a user-defined domain index cost function, if any. |
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Operation accepting a set of rows, eliminates some of them, and returns the rest. |
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Retrieval of only the first row selected by a query. |
|
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|
Operation retrieving and locking the rows selected by a query containing a |
|
(These are join operations.) |
|
Operation joining two sets of rows and returning the result. This join method is useful for joining large data sets of data (DSS, Batch). The join condition is an efficient way of accessing the second table. Query optimizer uses the smaller of the two tables/data sources to build a hash table on the join key in memory. Then it scans the larger table, probing the hash table to find the joined rows. |
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Hash (left) antijoin |
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Hash (left) semijoin |
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Hash right antijoin |
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Hash right semijoin |
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Hash (left) outer join |
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Hash right outer join |
|
(These are access methods.) |
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Retrieval of a single rowid from an index. |
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Retrieval of one or more rowids from an index. Indexed values are scanned in ascending order. |
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Retrieval of one or more rowids from an index. Indexed values are scanned in descending order. |
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Retrieval of all rowids from an index when there is no start or stop key. Indexed values are scanned in ascending order. |
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Retrieval of all rowids from an index when there is no start or stop key. Indexed values are scanned in descending order. |
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Retrieval of all rowids (and column values) using multiblock reads. No sorting order can be defined. Compares to a full table scan on only the indexed columns. Only available with the cost based optimizer. |
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Retrieval of rowids from a concatenated index without using the leading column(s) in the index. Introduced in Oracle9i. Only available with the cost based optimizer. |
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|
Iterates over the next operation in the plan for each value in the |
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Operation accepting two sets of rows and returning the intersection of the sets, eliminating duplicates. |
|
(These are join operations.) |
|
Operation accepting two sets of rows, each sorted by a specific value, combining each row from one set with the matching rows from the other, and returning the result. |
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Merge join operation to perform an outer join statement. |
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Merge antijoin. |
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Merge semijoin. |
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Can result from 1 or more of the tables not having any join conditions to any other tables in the statement. Can occur even with a join and it may not be flagged as |
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|
Retrieval of rows in hierarchical order for a query containing a |
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|
Operation accepting two sets of rows and returning rows appearing in the first set but not in the second, eliminating duplicates. |
|
(These are join operations.) |
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Operation accepting two sets of rows, an outer set and an inner set. Oracle compares each row of the outer set with each row of the inner set, returning rows that satisfy a condition. This join method is useful for joining small subsets of data (OLTP). The join condition is an efficient way of accessing the second table. |
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Nested loops operation to perform an outer join statement. |
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. |
Iterates over the next operation in the plan for each partition in the range given by the |
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|
Access one partition. |
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Access many partitions (a subset). |
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Access all partitions. |
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|
Similar to iterator, but based on an |
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Indicates that the partition set to be accessed is empty. |
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Implements the division of an object into block or chunk ranges among a set of parallel slaves |
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. |
Implements the Query Coordinator which controls, schedules, and executes the parallel plan below it using parallel query slaves. It also represents a serialization point, as the end of the part of the plan executed in parallel and always has a |
|
|
. |
Same semantics as the regular |
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|
. |
Shows the consumer/receiver slave node reading repartitioned data from a send/producer (QC or slave) executing on a PX SEND node. This information was formerly displayed into the |
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|
|
Implements the distribution method taking place between two parallel set of slaves. Shows the boundary between two slave sets and how data is repartitioned on the send/producer side (QC or side. This information was formerly displayed into the |
|
|
|
Retrieval of data from a remote database. |
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|
|
Operation involving accessing values of a sequence. |
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|
Retrieval of a single row that is the result of applying a group function to a group of selected rows. |
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|
Operation sorting a set of rows to eliminate duplicates. |
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|
Operation sorting a set of rows into groups for a query with a |
|
|
|
Operation sorting a set of rows before a merge-join. |
|
|
|
Operation sorting a set of rows for a query with an |
|
(These are access methods.) |
|
Retrieval of all rows from a table. |
|
|
|
Retrieval of sampled rows from a table. |
|
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|
Retrieval of rows from a table based on a value of an indexed cluster key. |
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|
Retrieval of rows from table based on hash cluster key value. |
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|
Retrieval of rows from a table based on a rowid range. |
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|
Retrieval of sampled rows from a table based on a rowid range. |
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If the table rows are located using user-supplied rowids. |
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|
If the table is nonpartitioned and rows are located using index(es). |
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If the table is partitioned and rows are located using only global indexes. |
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If the table is partitioned and rows are located using one or more local indexes and possibly some global indexes. The partition boundaries might have been computed by: A previous The | |
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|
Operation accepting two sets of rows and returns the union of the sets, eliminating duplicates. |
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Operation performing a view's query and then returning the resulting rows to another operation. |
| See Also:
Oracle Database Reference for more information on |
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 Copyright © 2000, 2003 Oracle Corporation All Rights Reserved. |
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