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Oracle® Database Performance Tuning Guide
11g Release 2 (11.2)

Part Number E16638-03
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12 Using EXPLAIN PLAN

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:

12.1 Understanding EXPLAIN PLAN

The EXPLAIN PLAN statement displays execution plans chosen by the optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement execution plan is the sequence of operations that the database performs to run the statement.

The row source tree is the core of the execution plan. The tree 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. The results also help 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.

12.1.1 How Execution Plans Can Change

With the query optimizer, execution plans can and do change as the underlying optimizer inputs change. EXPLAIN PLAN output shows how Oracle Database would run the SQL statement when the statement was explained. This plan can differ from the actual execution plan a SQL statement because of differences in the execution environment and explain plan environment.

Note:

To avoid possible SQL performance regression that may result from execution plan changes, consider using SQL plan management.

Execution plans can differ due to the following:

12.1.1.1 Different Schemas

  • The execution and explain plan occur on different databases.

  • The user explaining the statement is different from the user running the statement. Two users might be pointing to different objects in the same database, resulting in different execution plans.

  • Schema changes (usually changes in indexes) between the two operations.

12.1.1.2 Different Costs

Even if the schemas are the same, the optimizer can choose different execution plans when the costs are different. Some factors that affect the costs include the following:

  • Data volume and statistics

  • Bind variable types and values

  • Initialization parameters set globally or at session level

12.1.2 Minimizing Throw-Away

Examining an explain plan lets you look for throw-away in cases such as the following:

  • Full scans

  • Unselective range scans

  • Late predicate filters

  • Wrong join order

  • Late filter operations

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?

Example 12-1 Looking for Throw-Away in an Explain Plan

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)

12.1.3 Looking Beyond Execution Plans

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 are extremely inefficient. In this case, you should examine the following:

  • The columns of the index being used

  • Their selectivity (fraction of table being accessed)

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.

12.1.3.1 Using V$SQL_PLAN Views

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 shared SQL area. 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.

See Also:

12.1.4 EXPLAIN PLAN Restrictions

Oracle Database 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.

12.2 The PLAN_TABLE Output Table

The PLAN_TABLE is automatically created as a public synonym to a global temporary table. This temporary table holds the output of EXPLAIN PLAN statements 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 catplan.sql to manually create the global temporary table and the PLAN_TABLE synonym. The name and location of this script depends on your operating system. On UNIX and Linux, the script is located in the $ORACLE_HOME/rdbms/admin directory.

For example, start a SQL*Plus session, connect with SYSDBA privileges, and run the script as follows:

@$ORACLE_HOME/rdbms/admin/catplan.sql

Oracle 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 do not want to use the name PLAN_TABLE, create a new synonym after running the catplan.sql script. For example:

CREATE OR REPLACE PUBLIC SYNONYM my_plan_table for plan_table$

12.3 Running EXPLAIN PLAN

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".

12.3.1 Identifying Statements for EXPLAIN PLAN

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 12-2, st1 is specified as the statement identifier:

Example 12-2 Using EXPLAIN PLAN with the STATEMENT ID Clause

EXPLAIN PLAN
  SET STATEMENT_ID = 'st1' FOR
SELECT last_name FROM employees;

12.3.2 Specifying Different Tables for EXPLAIN PLAN

You can specify the INTO clause to specify a different table.

Example 12-3 Using EXPLAIN PLAN with the INTO Clause

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 Language Reference for a complete description of EXPLAIN PLAN syntax.

12.4 Displaying PLAN_TABLE Output

After you have explained the plan, use the following SQL scripts or PL/SQL package provided by Oracle Database to display the most recent plan table output:

12.4.1 Customizing PLAN_TABLE Output

If you have specified a statement identifier, then you can write your own script to query the PLAN_TABLE. For example:

  • Start with ID = 0 and given STATEMENT_ID.

  • Use the 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.

  • Use the pseudo-column 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.

    Note:

    These simplified examples are not valid for recursive SQL.

12.5 Reading EXPLAIN PLAN Output

This section uses EXPLAIN PLAN examples to illustrate execution plans. The statement in Example 12-4 displays the execution plans.

Example 12-4 Statement to display the EXPLAIN PLAN

SELECT PLAN_TABLE_OUTPUT 
  FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'statement_id','BASIC'));

Examples of the output from this statement are shown in Example 12-5 and Example 12-6.

Example 12-5 EXPLAIN PLAN for Statement ID ex_plan1

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.

Example 12-6 EXPLAIN PLAN for Statement ID ex_plan2

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.

12.6 Viewing Parallel Execution with EXPLAIN PLAN

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.

Figure 12-1 A Table Hierarchy

Description of Figure 12-1 follows
Description of "Figure 12-1 A Table Hierarchy"

CUSTOMER is the smallest table while TRANSACTION is the largest. A typical OLTP query might retrieve transaction information about a specific 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 the database can use parallel query. It would not be efficient to use parallel query in this case 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 database would join rows from the TRANSACTION table to the ACCOUNT table, and finally to the CUSTOMER table. In this case, the indexes used on the ACCOUNT and CUSTOMER table are probably 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:

If a parallel step produces many rows, then the QC may not be able to consume the rows as fast as they are produced. Little can be done to improve this situation.

See Also:

See the OTHER_TAG column in Table 12-1, "PLAN_TABLE Columns"

12.6.1 Viewing Parallel Queries with EXPLAIN PLAN

When using EXPLAIN PLAN with parallel queries, the database compiles and executes one parallel plan. This plan is derived from the serial plan by allocating row sources specific to the parallel support in the 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 12-7 is a simple query for illustrating an EXPLAIN PLAN for a parallel query.

Example 12-7 Parallel Query Explain Plan

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 (%CPU) |    TQ  |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |          |   107 |  2782 |     3 (34)  |        |      |            |
|   1 |  PX COORDINATOR          |          |       |       |             |        |      |            |
|   2 |   PX SEND QC (RANDOM)    | :TQ10001 |   107 |  2782 |     3 (34)  |  Q1,01 | P->S | QC (RAND)  |
|   3 |    HASH GROUP BY         |          |   107 |  2782 |     3 (34)  |  Q1,01 | PCWP |            |
|   4 |     PX RECEIVE           |          |   107 |  2782 |     3 (34)  |  Q1,01 | PCWP |            |
|   5 |      PX SEND HASH        | :TQ10000 |   107 |  2782 |     3 (34)  |  Q1,00 | P->P | HASH       |
|   6 |       HASH GROUP BY      |          |   107 |  2782 |     3 (34)  |  Q1,00 | PCWP |            |
|   7 |        PX BLOCK ITERATOR |          |   107 |  2782 |     2 (0)   |  Q1,00 | PCWP |            |
|   8 |         TABLE ACCESS FULL| EMP2     |   107 |  2782 |     2 (0)   |  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 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.

12.7 Viewing Bitmap Indexes with EXPLAIN PLAN

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 12-8.

Example 12-8 EXPLAIN PLAN with Bitmap Indexes

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 generates the rowids necessary for the table access.

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.

12.8 Viewing Result Cache with EXPLAIN PLAN

When your query contains the result_cache hint, the ResultCache operator is inserted into the execution plan.

For example, consider the query:

select /*+ result_cache */ deptno, avg(sal) 
from emp 
group by deptno;

To view the EXPLAIN PLAN for this query, use the command:

EXPLAIN PLAN FOR 
select /*+ result_cache */ deptno, avg(sal) 
from emp 
group by deptno;
select PLAN_TABLE_OUTPUT from TABLE (DBMS_XPLAN.DISPLAY());

The EXPLAIN PLAN output for this query should look similar to the following:

---------------------------------------------------------------------------------------------
| Id  | Operation           | Name                        |Rows |Bytes |Cost(%CPU)|Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |                             | 11  | 77   |  4 (25)  | 00:00:01|
|   1 |  RESULT CACHE       |b06ppfz9pxzstbttpbqyqnfbmy   |     |      |          |         |
|   2 |   HASH GROUP BY     |                             | 11  | 77   |  4 (25)  | 00:00:01|
|   3 |    TABLE ACCESS FULL| EMP                         |107  | 749  |  3 (0)   | 00:00:01|
---------------------------------------------------------------------------------------------

In this EXPLAIN PLAN, the ResultCache operator is identified by its CacheId, which is b06ppfz9pxzstbttpbqyqnfbmy. You can now run a query on the V$RESULT_CACHE_OBJECTS view by using this CacheId.

12.9 Viewing Partitioned Objects with EXPLAIN PLAN

Use EXPLAIN PLAN to see how Oracle Database 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.

12.9.1 Examples of Displaying Range and Hash Partitioning with EXPLAIN PLAN

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 Database 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 Database 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 |
---------------------------------------------------------------------------------

The database creates a partition row source 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 the database prunes 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.

12.9.1.1 Plans for Hash Partitioning

Oracle Database 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.

12.9.2 Examples of Pruning Information with Composite Partitioned Objects

To illustrate how Oracle Database 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 Database accesses all subpartitions of all partitions of a composite object. The database uses two partition row sources for this 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 the database performs no pruning. 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 the database does 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 Database 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 Database accesses only one subpartition within each partition. The PARTITION_START and PARTITION_STOP is set to KEY, which means that Oracle Database determines the number of subpartitions at run time.

12.9.3 Examples of Partial Partition-Wise Joins

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 Database dynamically partitions the dept2 table before the join.

Example 12-9 Partial Partition-Wise Join with Range Partition

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. The database dynamically partitions the dept2 table before the join.

Example 12-10 Partial Partition-Wise Join with Composite Partition

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 12-9 and Example 12-10 the PQ_DISTRIBUTE hint explicitly forces a partial partition-wise join because the query optimizer could have chosen a different plan based on cost in this query.

12.9.4 Examples of Full Partition-wise Joins

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.

Example 12-11 Full Partition-Wise Join

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 |            |
-------------------------------------------------------------------------------------------------------------

12.9.5 Examples of INLIST ITERATOR and EXPLAIN PLAN

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.

12.9.5.1 When the IN-List Column is an Index Column

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 and stop keys.

12.9.5.2 When the IN-List Column is an Index and a Partition Column

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)

12.9.5.3 When the IN-List Column is a Partition Column

If empno is a partition column and no indexes exist, 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

12.9.6 Example of Domain Indexes and EXPLAIN PLAN

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. You explain the plan for the following query:

SELECT * FROM emp WHERE CONTAINS(resume, 'Oracle') = 1 

The database could 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

12.10 PLAN_TABLE Columns

The PLAN_TABLE used by the EXPLAIN PLAN statement contains the columns listed in Table 12-1.

Table 12-1 PLAN_TABLE Columns

Column Type Description

STATEMENT_ID

VARCHAR2(30)

Value of the optional STATEMENT_ID parameter specified in the EXPLAIN PLAN statement.

PLAN_ID

NUMBER

Unique identifier of a plan in the database.

TIMESTAMP

DATE

Date and time when the EXPLAIN PLAN statement was generated.

REMARKS

VARCHAR2(80)

Any comment (of up to 80 bytes) you want to associate with each step of the explained plan. This column indicates whether the database used an outline or SQL profile for the query.

If you need to add or change a remark on any row of the PLAN_TABLE, then use the UPDATE statement to modify the rows of the PLAN_TABLE.

OPERATION

VARCHAR2(30)

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:

  • DELETE STATEMENT

  • INSERT STATEMENT

  • SELECT STATEMENT

  • UPDATE STATEMENT

See Table 12-3 for more information on values for this column.

OPTIONS

VARCHAR2(225)

A variation on the operation described in the OPERATION column.

See Table 12-3 for more information on values for this column.

OBJECT_NODE

VARCHAR2(128)

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 the database consumes output from operations.

OBJECT_OWNER

VARCHAR2(30)

Name of the user who owns the schema containing the table or index.

OBJECT_NAME

VARCHAR2(30)

Name of the table or index.

OBJECT_ALIAS

VARCHAR2(65)

Unique alias of a table or view in a SQL statement. For indexes, it is the object alias of the underlying table.

OBJECT_INSTANCE

NUMERIC

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 for the original statement text. View expansion results in unpredictable numbers.

OBJECT_TYPE

VARCHAR2(30)

Modifier that provides descriptive information about the object; for example, NON-UNIQUE for indexes.

OPTIMIZER

VARCHAR2(255)

Current mode of the optimizer.

SEARCH_COLUMNS

NUMBERIC

Not currently used.

ID

NUMERIC

A number assigned to each step in the execution plan.

PARENT_ID

NUMERIC

The ID of the next execution step that operates on the output of the ID step.

DEPTH

NUMERIC

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.

POSITION

NUMERIC

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.

COST

NUMERIC

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 CPU_COST and IO_COST columns.

CARDINALITY

NUMERIC

Estimate by the query optimization approach of the number of rows accessed by the operation.

BYTES

NUMERIC

Estimate by the query optimization approach of the number of bytes accessed by the operation.

OTHER_TAG

VARCHAR2(255)

Describes the contents of the OTHER column. Values are:

  • SERIAL (blank): Serial execution. Currently, SQL is not loaded in the OTHER column for this case.

  • SERIAL_FROM_REMOTE (S -> R): Serial execution at a remote site.

  • PARALLEL_FROM_SERIAL (S -> P): Serial execution. Output of step is partitioned or broadcast to parallel execution servers.

  • PARALLEL_TO_SERIAL (P -> S): Parallel execution. Output of step is returned to serial QC process.

  • PARALLEL_TO_PARALLEL (P -> P): Parallel execution. Output of step is repartitioned to second set of parallel execution servers.

  • PARALLEL_COMBINED_WITH_PARENT (PWP): Parallel execution; Output of step goes to next step in same parallel process. No interprocess communication to parent.

  • PARALLEL_COMBINED_WITH_CHILD (PWC): Parallel execution. Input of step comes from prior step in same parallel process. No interprocess communication from child.

PARTITION_START

VARCHAR2(255)

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.

KEY indicates that the start partition is identified at run time from partitioning key values.

ROW REMOVE_LOCATION indicates that the database computes the start partition (same as the stop partition) at run time from the location of each retrieved record. The record location is obtained by a user or from a global index.

INVALID indicates that the range of accessed partitions is empty.

PARTITION_STOP

VARCHAR2(255)

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.

KEY indicates that the stop partition is identified at run time from partitioning key values.

ROW REMOVE_LOCATION indicates that the database computes the stop partition (same as the start partition) at run time from the location of each retrieved record. The record location is obtained by a user or from a global index.

INVALID indicates that the range of accessed partitions is empty.

PARTITION_ID

NUMERIC

Step that has computed the pair of values of the PARTITION_START and PARTITION_STOP columns.

OTHER

LONG

Other information that is specific to the execution step that a user might find useful. See the OTHER_TAG column.

DISTRIBUTION

VARCHAR2(30)

Method used to distribute rows from producer query servers to consumer query servers.

See Table 12-2 for more information on the possible values for this column. For more information about consumer and producer query servers, see Oracle Database Data Warehousing Guide.

CPU_COST

NUMERIC

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.

IO_COST

NUMERIC

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.

TEMP_SPACE

NUMERIC

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.

ACCESS_PREDICATES

VARCHAR2(4000)

Predicates used to locate rows in an access structure. For example, start or stop predicates for an index range scan.

FILTER_PREDICATES

VARCHAR2(4000)

Predicates used to filter rows before producing them.

PROJECTION

VARCHAR2(4000)

Expressions produced by the operation.

TIME

NUMBER(20,2)

Elapsed time in seconds of the operation as estimated by query optimization. For statements that use the rule-based approach, this column is null.

QBLOCK_NAME

VARCHAR2(30)

Name of the query block, either system-generated or defined by the user with the QB_NAME hint.


Table 12-2 describes the values that can appear in the DISTRIBUTION column:

Table 12-2 Values of DISTRIBUTION Column of the PLAN_TABLE

DISTRIBUTION Text Interpretation

PARTITION (ROWID)

Maps rows to query servers based on the partitioning of a table or index using the rowid of the row to UPDATE/DELETE.

PARTITION (KEY)

Maps rows to query servers based on the partitioning of a table or index using a set of columns. Used for partial partition-wise join, PARALLEL INSERT, CREATE TABLE AS SELECT of a partitioned table, and CREATE PARTITIONED GLOBAL INDEX.

HASH

Maps rows to query servers using a hash function on the join key. Used for PARALLEL JOIN or PARALLEL GROUP BY.

RANGE

Maps rows to query servers using ranges of the sort key. Used when the statement contains an ORDER BY clause.

ROUND-ROBIN

Randomly maps rows to query servers.

BROADCAST

Broadcasts the rows of the entire table to each query server. Used for a parallel join when one table is very small compared to the other.

QC (ORDER)

The QC consumes the input in order, from the first to the last query server. Used when the statement contains an ORDER BY clause.

QC (RANDOM)

The QC consumes the input randomly. Used when the statement does not have an ORDER BY clause.


Table 12-3 lists each combination of OPERATION and OPTIONS produced by the EXPLAIN PLAN statement and its meaning within an execution plan.

Table 12-3 OPERATION and OPTIONS Values Produced by EXPLAIN PLAN

Operation Option Description

AND-EQUAL

 

Operation accepting multiple sets of rowids, returning the intersection of the sets, eliminating duplicates. Used for the single-column indexes access path.

BITMAP

CONVERSION

TO ROWIDS converts bitmap representations to actual rowids that can be used to access the table.

FROM ROWIDS converts the rowids to a bitmap representation.

COUNT returns the number of rowids if the actual values are not needed.

BITMAP

INDEX

SINGLE VALUE looks up the bitmap for a single key value in the index.

RANGE SCAN retrieves bitmaps for a key value range.

FULL SCAN performs a full scan of a bitmap index if there is no start or stop key.

BITMAP

MERGE

Merges several bitmaps resulting from a range scan into one bitmap.

BITMAP

MINUS

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".

BITMAP

OR

Computes the bitwise OR of two bitmaps.

BITMAP

AND

Computes the bitwise AND of two bitmaps.

BITMAP

KEY ITERATION

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 BITMAP MERGE operation.

CONNECT BY

 

Retrieves rows in hierarchical order for a query containing a CONNECT BY clause.

CONCATENATION

 

Operation accepting multiple sets of rows returning the union-all of the sets.

COUNT

 

Operation counting the number of rows selected from a table.

COUNT

STOPKEY

Count operation where the number of rows returned is limited by the ROWNUM expression in the WHERE clause.

CUBE SCAN

 

Uses inner joins for all cube access.

CUBE SCAN

PARTIAL OUTER

Uses an outer join for at least one dimension, and inner joins for the other dimensions.

CUBE SCAN

OUTER

Uses outer joins for all cube access.

DOMAIN INDEX

 

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.

FILTER

 

Operation accepting a set of rows, eliminates some of them, and returns the rest.

FIRST ROW

 

Retrieval of only the first row selected by a query.

FOR UPDATE

 

Operation retrieving and locking the rows selected by a query containing a FOR UPDATE clause.

HASH

GROUP BY

Operation hashing a set of rows into groups for a query with a GROUP BY clause.

HASH

GROUP BY PIVOT

Operation hashing a set of rows into groups for a query with a GROUP BY clause. The PIVOT option indicates a pivot-specific optimization for the HASH GROUP BY operator.

HASH JOIN

(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.

HASH JOIN

ANTI

Hash (left) antijoin

HASH JOIN

SEMI

Hash (left) semijoin

HASH JOIN

RIGHT ANTI

Hash right antijoin

HASH JOIN

RIGHT SEMI

Hash right semijoin

HASH JOIN

OUTER

Hash (left) outer join

HASH JOIN

RIGHT OUTER

Hash right outer join

INDEX

(These are access methods.)

UNIQUE SCAN

Retrieval of a single rowid from an index.

INDEX

RANGE SCAN

Retrieval of one or more rowids from an index. Indexed values are scanned in ascending order.

INDEX

RANGE SCAN DESCENDING

Retrieval of one or more rowids from an index. Indexed values are scanned in descending order.

INDEX

FULL SCAN

Retrieval of all rowids from an index when there is no start or stop key. Indexed values are scanned in ascending order.

INDEX

FULL SCAN DESCENDING

Retrieval of all rowids from an index when there is no start or stop key. Indexed values are scanned in descending order.

INDEX

FAST FULL SCAN

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.

INDEX

SKIP SCAN

Retrieval of rowids from a concatenated index without using the leading column(s) in the index. Only available with the cost based optimizer.

INLIST ITERATOR

 

Iterates over the next operation in the plan for each value in the IN-list predicate.

INTERSECTION

 

Operation accepting two sets of rows and returning the intersection of the sets, eliminating duplicates.

MERGE JOIN

(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.

MERGE JOIN

OUTER

Merge join operation to perform an outer join statement.

MERGE JOIN

ANTI

Merge antijoin.

MERGE JOIN

SEMI

Merge semijoin.

MERGE JOIN

CARTESIAN

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 CARTESIAN in the plan.

CONNECT BY

 

Retrieval of rows in hierarchical order for a query containing a CONNECT BY clause.

MAT_VIEW REWITE ACCESS

(These are access methods.)

FULL

Retrieval of all rows from a materialized view.

MAT_VIEW REWITE ACCESS

SAMPLE

Retrieval of sampled rows from a materialized view.

MAT_VIEW REWITE ACCESS

CLUSTER

Retrieval of rows from a materialized view based on a value of an indexed cluster key.

MAT_VIEW REWITE ACCESS

HASH

Retrieval of rows from materialized view based on hash cluster key value.

MAT_VIEW REWITE ACCESS

BY ROWID RANGE

Retrieval of rows from a materialized view based on a rowid range.

MAT_VIEW REWITE ACCESS

SAMPLE BY ROWID RANGE

Retrieval of sampled rows from a materialized view based on a rowid range.

MAT_VIEW REWITE ACCESS

BY USER ROWID

If the materialized view rows are located using user-supplied rowids.

MAT_VIEW REWITE ACCESS

BY INDEX ROWID

If the materialized view is nonpartitioned and rows are located using index(es).

MAT_VIEW REWITE ACCESS

BY GLOBAL INDEX ROWID

If the materialized view is partitioned and rows are located using only global indexes.

MAT_VIEW REWITE ACCESS

BY LOCAL INDEX ROWID

If the materialized view is partitioned and rows are located using one or more local indexes and possibly some global indexes.

Partition Boundaries:

The partition boundaries might have been computed by:

A previous PARTITION step, in which case the PARTITION_START and PARTITION_STOP column values replicate the values present in the PARTITION step, and the PARTITION_ID contains the ID of the PARTITION step. Possible values for PARTITION_START and PARTITION_STOP are NUMBER(n), KEY, INVALID.

The MAT_VIEW REWRITE ACCESS or INDEX step itself, in which case the PARTITION_ID contains the ID of the step. Possible values for PARTITION_START and PARTITION_STOP are NUMBER(n), KEY, ROW REMOVE_LOCATION (MAT_VIEW REWRITE ACCESS only), and INVALID.

MINUS

 

Operation accepting two sets of rows and returning rows appearing in the first set but not in the second, eliminating duplicates.

NESTED LOOPS

(These are join operations.)

 

Operation accepting two sets of rows, an outer set and an inner set. Oracle Database 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.

NESTED LOOPS

OUTER

Nested loops operation to perform an outer join statement.

PARTITION

 

Iterates over the next operation in the plan for each partition in the range given by the PARTITION_START and PARTITION_STOP columns. PARTITION describes partition boundaries applicable to a single partitioned object (table or index) or to a set of equi-partitioned objects (a partitioned table and its local indexes). The partition boundaries are provided by the values of PARTITION_START and PARTITION_STOP of the PARTITION. Refer to Table 12-1 for valid values of partition start and stop.

PARTITION

SINGLE

Access one partition.

PARTITION

ITERATOR

Access many partitions (a subset).

PARTITION

ALL

Access all partitions.

PARTITION

INLIST

Similar to iterator, but based on an IN-list predicate.

PARTITION

INVALID

Indicates that the partition set to be accessed is empty.

PX ITERATOR

BLOCK, CHUNK

Implements the division of an object into block or chunk ranges among a set of parallel slaves.

PX COORDINATOR

 

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 PX SEND QC operation below it.

PX PARTITION

 

Same semantics as the regular PARTITION operation except that it appears in a parallel plan.

PX RECEIVE

 

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 DISTRIBUTION column. See Table 12-2.

PX SEND

QC (RANDOM), HASH, RANGE

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 DISTRIBUTION column. See Table 12-2.

REMOTE

 

Retrieval of data from a remote database.

SEQUENCE

 

Operation involving accessing values of a sequence.

SORT

AGGREGATE

Retrieval of a single row that is the result of applying a group function to a group of selected rows.

SORT

UNIQUE

Operation sorting a set of rows to eliminate duplicates.

SORT

GROUP BY

Operation sorting a set of rows into groups for a query with a GROUP BY clause.

SORT

GROUP BY PIVOT

Operation sorting a set of rows into groups for a query with a GROUP BY clause. The PIVOT option indicates a pivot-specific optimization for the SORT GROUP BY operator.

SORT

JOIN

Operation sorting a set of rows before a merge-join.

SORT

ORDER BY

Operation sorting a set of rows for a query with an ORDER BY clause.

TABLE ACCESS

(These are access methods.)

FULL

Retrieval of all rows from a table.

TABLE ACCESS

SAMPLE

Retrieval of sampled rows from a table.

TABLE ACCESS

CLUSTER

Retrieval of rows from a table based on a value of an indexed cluster key.

TABLE ACCESS

HASH

Retrieval of rows from table based on hash cluster key value.

TABLE ACCESS

BY ROWID RANGE

Retrieval of rows from a table based on a rowid range.

TABLE ACCESS

SAMPLE BY ROWID RANGE

Retrieval of sampled rows from a table based on a rowid range.

TABLE ACCESS

BY USER ROWID

If the table rows are located using user-supplied rowids.

TABLE ACCESS

BY INDEX ROWID

If the table is nonpartitioned and rows are located using index(es).

TABLE ACCESS

BY GLOBAL INDEX ROWID

If the table is partitioned and rows are located using only global indexes.

TABLE ACCESS

BY LOCAL INDEX ROWID

If the table is partitioned and rows are located using one or more local indexes and possibly some global indexes.

Partition Boundaries:

The partition boundaries might have been computed by:

A previous PARTITION step, in which case the PARTITION_START and PARTITION_STOP column values replicate the values present in the PARTITION step, and the PARTITION_ID contains the ID of the PARTITION step. Possible values for PARTITION_START and PARTITION_STOP are NUMBER(n), KEY, INVALID.

The TABLE ACCESS or INDEX step itself, in which case the PARTITION_ID contains the ID of the step. Possible values for PARTITION_START and PARTITION_STOP are NUMBER(n), KEY, ROW REMOVE_LOCATION (TABLE ACCESS only), and INVALID.

TRANSPOSE

 

Operation evaluating a PIVOT operation by transposing the results of GROUP BY to produce the final pivoted data.

UNION

 

Operation accepting two sets of rows and returns the union of the sets, eliminating duplicates.

UNPIVOT

 

Operation that rotates data from columns into rows.

VIEW

 

Operation performing a view's query and then returning the resulting rows to another operation.


See Also:

Oracle Database Reference for more information on PLAN_TABLE