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Oracle® Database VLDB and Partitioning Guide
12c Release 1 (12.1)

E17613-20
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Miscellaneous Parallel Execution Tuning Tips

This section contains some ideas for improving performance in a parallel execution environment and includes the following topics:

Creating and Populating Tables in Parallel

Oracle Database cannot return results to a user process in parallel. If a query returns a large number of rows, execution of the query might indeed be faster. However, the user process can receive the rows only serially. To optimize parallel execution performance for queries that retrieve large result sets, use PARALLEL CREATE TABLE AS SELECT or direct-path INSERT to store the result set in the database. At a later time, users can view the result set serially.

Performing the SELECT in parallel does not influence the CREATE statement. If the CREATE statement is executed in parallel, however, the optimizer tries to make the SELECT run in parallel also.

When combined with the NOLOGGING option, the parallel version of CREATE TABLE AS SELECT provides a very efficient intermediate table facility, for example:

CREATE TABLE summary PARALLEL NOLOGGING AS SELECT dim_1, dim_2 ..., 
SUM (meas_1)
FROM facts GROUP BY dim_1, dim_2;

These tables can also be incrementally loaded with parallel INSERT. You can take advantage of intermediate tables using the following techniques:

  • Common subqueries can be computed once and referenced many times. This can allow some queries against star schemas (in particular, queries without selective WHERE-clause predicates) to be better parallelized. Star queries with selective WHERE-clause predicates using the star-transformation technique can be effectively parallelized automatically without any modification to the SQL.

  • Decompose complex queries into simpler steps to provide application-level checkpoint or restart. For example, a complex multitable join on a one terabyte database could run for dozens of hours. A failure during this query would mean starting over from the beginning. Using CREATE TABLE AS SELECT or PARALLEL INSERT AS SELECT, you can rewrite the query as a sequence of simpler queries that run for a few hours each. If a system failure occurs, the query can be restarted from the last completed step.

  • Implement manual parallel delete operations efficiently by creating a new table that omits the unwanted rows from the original table, and then dropping the original table. Alternatively, you can use the convenient parallel delete feature, which directly deletes rows from the original table.

  • Create summary tables for efficient multidimensional drill-down analysis. For example, a summary table might store the sum of revenue grouped by month, brand, region, and salesman.

  • Reorganize tables, eliminating chained rows, compressing free space, and so on, by copying the old table to a new table. This is much faster than export/import and easier than reloading.

Be sure to use the DBMS_STATS package to gather optimizer statistics on newly created tables. To avoid I/O bottlenecks, specify a tablespace that is striped across at least as many physical disks as CPUs. To avoid fragmentation in allocating space, the number of files in a tablespace should be a multiple of the number of CPUs.

See Also:

Oracle Database Data Warehousing Guide for information about parallel execution in data warehouses

Using EXPLAIN PLAN to Show Parallel Operations Plans

Use the EXPLAIN PLAN statement to see the execution plans for parallel queries. The EXPLAIN PLAN output shows optimizer information in the COST, BYTES, and CARDINALITY columns. You can also use the utlxplp.sql script to present the EXPLAIN PLAN output with all relevant parallel information.

There are several ways to optimize the parallel execution of join statements. You can alter system configuration, adjust parameters as discussed earlier in this chapter, or use hints, such as the DISTRIBUTION hint.

The key points when using EXPLAIN PLAN are to:

  • Verify optimizer selectivity estimates. If the optimizer thinks that only one row is produced from a query, it tends to favor using a nested loop. This could be an indication that the tables are not analyzed or that the optimizer has made an incorrect estimate about the correlation of multiple predicates on the same table. Extended statistics or a hint may be required to provide the optimizer with the correct selectivity or to force the optimizer to use another join method.

  • Use hash join on low cardinality join keys. If a join key has few distinct values, then a hash join may not be optimal. If the number of distinct values is less than the degree of parallelism (DOP), then some parallel query servers may be unable to work on the particular query.

  • Consider data skew. If a join key involves excessive data skew, a hash join may require some parallel query servers to work more than others. Consider using a hint to cause a BROADCAST distribution method if the optimizer did not choose it. The optimizer considers the BROADCAST distribution method only if the OPTIMIZER_FEATURES_ENABLE is set to 9.0.2 or higher. See "V$PQ_TQSTAT" for more information.

Example: Using EXPLAIN PLAN to Show Parallel Operations

The following example illustrates how the optimizer intends to execute a parallel query:

explain plan for 
SELECT /*+ PARALLEL */ cust_first_name, cust_last_name 
FROM customers c, sales s WHERE c.cust_id = s.cust_id;

----------------------------------------------------------
| Id  | Operation                       |  Name          |
----------------------------------------------------------
|   0 | SELECT STATEMENT                |                |
|   1 |  PX COORDINATOR                 |                |
|   2 |   PX SEND QC (RANDOM)           | :TQ10000       |
|   3 |    NESTED LOOPS                 |                |
|   4 |     PX BLOCK ITERATOR           |                |
|   5 |      TABLE ACCESS FULL          | CUSTOMERS      |
|   6 |     PARTITION RANGE ALL         |                |
|   7 |      BITMAP CONVERSION TO ROWIDS|                |
|   8 |       BITMAP INDEX SINGLE VALUE | SALES_CUST_BIX |
----------------------------------------------------------

Note
-----
   - Computed Degree of Parallelism is 2
   - Degree of Parallelism of 2 is derived from scan of object SH.CUSTOMERS

Additional Considerations for Parallel DML

When you want to refresh your data warehouse database using parallel insert, update, or delete operations on a data warehouse, there are additional issues to consider when designing the physical database. These considerations do not affect parallel execution operations. These issues are:

Parallel DML and Direct-Path Restrictions

If a parallel restriction is violated, the operation is simply performed serially. If a direct-path INSERT restriction is violated, then the APPEND hint is ignored and a conventional insert operation is performed. No error message is returned.

Limitation on the Degree of Parallelism

For tables that do not have the parallel DML itl invariant property (tables created before Oracle9i Release 2 (9.2) or tables that were created with the COMPATIBLE initialization parameter set to less than 9.2), the degree of parallelism (DOP) equals the number of partitions or subpartitions. That means that, if the table is not partitioned, the query runs serially. To see what tables do not have this property, issue the following statement:

SELECT u.name, o.name FROM obj$ o, tab$ t, user$ u
WHERE o.obj# = t.obj# AND o.owner# = u.user#
 AND bitand(t.property,536870912) != 536870912;

For information about the interested transaction list (ITL), also called the transaction table, refer to Oracle Database Concepts.

Increasing INITRANS

If you have global indexes, a global index segment and global index blocks are shared by server processes of the same parallel DML statement. Even if the operations are not performed against the same row, the server processes can share the same index blocks. Each server transaction needs one transaction entry in the index block header before it can make changes to a block. Therefore, in the CREATE INDEX or ALTER INDEX statements, you should set INITRANS, the initial number of transactions allocated within each data block, to a large value, such as the maximum DOP against this index.

Limitation on Available Number of Transaction Free Lists for Segments

There is a limitation on the available number of transaction free lists for segments in dictionary-managed tablespaces. After a segment has been created, the number of process and transaction free lists is fixed and cannot be altered. If you specify a large number of process free lists in the segment header, you might find that this limits the number of transaction free lists that are available. You can abate this limitation the next time you re-create the segment header by decreasing the number of process free lists; this leaves more room for transaction free lists in the segment header.

For UPDATE and DELETE operations, each server process can require its own transaction free list. The parallel DML DOP is thus effectively limited by the smallest number of transaction free lists available on the table and on any of the global indexes the DML statement must maintain. For example, if the table has 25 transaction free lists and the table has two global indexes, one with 50 transaction free lists and one with 30 transaction free lists, the DOP is limited to 25. If the table had 40 transaction free lists, the DOP would have been limited to 30.

The FREELISTS parameter of the STORAGE clause is used to set the number of process free lists. By default, no process free lists are created.

The default number of transaction free lists depends on the block size. For example, if the number of process free lists is not set explicitly, a 4 KB block has about 80 transaction free lists by default. The minimum number of transaction free lists is 25.

Using Multiple Archivers

Parallel DDL and parallel DML operations can generate a large number of redo logs. A single ARCH process to archive these redo logs might not be able to keep up. To avoid this problem, you can spawn multiple archiver processes manually or by using a job queue.

Database Writer Process (DBWn) Workload

Parallel DML operations use a large number of data, index, and undo blocks in the buffer cache during a short interval. For example, suppose you see a high number of free_buffer_waits after querying the V$SYSTEM_EVENT view, as in the following syntax:

SELECT TOTAL_WAITS FROM V$SYSTEM_EVENT WHERE EVENT = 'FREE BUFFER WAITS';

In this case, you should consider increasing the DBWn processes. If there are no waits for free buffers, the query does not return any rows.

[NO]LOGGING Clause

The [NO]LOGGING clause applies to tables, partitions, tablespaces, and indexes. Virtually no log is generated for certain operations (such as direct-path INSERT) if the NOLOGGING clause is used. The NOLOGGING attribute is not specified at the INSERT statement level but is instead specified when using the ALTER or CREATE statement for a table, partition, index, or tablespace.

When a table or index has NOLOGGING set, neither parallel nor serial direct-path INSERT operations generate redo logs. Processes running with the NOLOGGING option set run faster because no redo is generated. However, after a NOLOGGING operation against a table, partition, or index, if a media failure occurs before a backup is performed, then all tables, partitions, and indexes that have been modified might be corrupted.

Direct-path INSERT operations (except for dictionary updates) never generate redo logs if the NOLOGGING clause is used. The NOLOGGING attribute does not affect undo, only redo. To be precise, NOLOGGING allows the direct-path INSERT operation to generate a negligible amount of redo (range-invalidation redo, as opposed to full image redo).

For backward compatibility, [UN]RECOVERABLE is still supported as an alternate keyword with the CREATE TABLE statement. This alternate keyword might not be supported, however, in future releases.

At the tablespace level, the logging clause specifies the default logging attribute for all tables, indexes, and partitions created in the tablespace. When an existing tablespace logging attribute is changed by the ALTER TABLESPACE statement, then all tables, indexes, and partitions created after the ALTER statement have the new logging attribute; existing ones do not change their logging attributes. The tablespace-level logging attribute can be overridden by the specifications at the table, index, or partition level.

The default logging attribute is LOGGING. However, if you have put the database in NOARCHIVELOG mode, by issuing ALTER DATABASE NOARCHIVELOG, then all operations that can be done without logging do not generate logs, regardless of the specified logging attribute.

Creating Indexes in Parallel

Multiple processes can work simultaneously to create an index. By dividing the work necessary to create an index among multiple server processes, Oracle Database can create the index more quickly than if a single server process created the index serially.

Parallel index creation works in much the same way as a table scan with an ORDER BY clause. The table is randomly sampled and a set of index keys is found that equally divides the index into the same number of pieces as the DOP. A first set of query processes scans the table, extracts key-rowid pairs, and sends each pair to a process in a second set of query processes based on a key. Each process in the second set sorts the keys and builds an index in the usual fashion. After all index pieces are built, the parallel execution coordinator simply concatenates the pieces (which are ordered) to form the final index.

Parallel local index creation uses a single server set. Each server process in the set is assigned a table partition to scan and for which to build an index partition. Because half as many server processes are used for a given DOP, parallel local index creation can be run with a higher DOP. However, the DOP is restricted to be less than or equal to the number of index partitions you want to create. To avoid this limitation, you can use the DBMS_PCLXUTIL package.

You can optionally specify that no redo and undo logging should occur during index creation. This can significantly improve performance but temporarily renders the index unrecoverable. Recoverability is restored after the new index is backed up. If your application can tolerate a window where recovery of the index requires it to be re-created, then you should consider using the NOLOGGING clause.

The PARALLEL clause in the CREATE INDEX statement is the only way in which you can specify the DOP for creating the index. If the DOP is not specified in the parallel clause of the CREATE INDEX statement, then the number of CPUs is used as the DOP. If there is no PARALLEL clause, index creation is done serially.

When creating an index in parallel, the STORAGE clause refers to the storage of each of the subindexes created by the query server processes. Therefore, an index created with an INITIAL value of 5 MB and a DOP of 12 consumes at least 60 MB of storage during index creation because each process starts with an extent of 5 MB. When the query coordinator process combines the sorted subindexes, some extents might be trimmed, and the resulting index might be smaller than the requested 60 MB.

When you add or enable a UNIQUE or PRIMARY KEY constraint on a table, you cannot automatically create the required index in parallel. Instead, manually create an index on the desired columns, using the CREATE INDEX statement and an appropriate PARALLEL clause, and then add or enable the constraint. Oracle Database then uses the existing index when enabling or adding the constraint.

Multiple constraints on the same table can be enabled concurrently and in parallel if all the constraints are in the ENABLE NOVALIDATE state. In the following example, the ALTER TABLE ENABLE CONSTRAINT statement performs the table scan that checks the constraint in parallel:

CREATE TABLE a (a1 NUMBER CONSTRAINT ach CHECK (a1 > 0) ENABLE NOVALIDATE)
PARALLEL; 
INSERT INTO a values (1);
COMMIT;
ALTER TABLE a ENABLE CONSTRAINT ach;

Parallel DML Tips

This section provides an overview of parallel DML functionality. The topics covered include:

See Also:

Parallel DML Tip 1: INSERT

The functionality available using an INSERT statement can be summarized as shown in Table 8-5:

Table 8-5 Summary of INSERT Features

Insert Type Parallel Serial NOLOGGING

Conventional

No

See text in this section for information about using the NOAPPEND hint with parallel DML enabled to perform a parallel conventional insert.

Yes

No

Direct-path

INSERT

(APPEND)

Yes, but requires

ALTER SESSION ENABLE PARALLEL DML to enable PARALLEL DML mode

and one of the following:

  • Table PARALLEL attribute or PARALLEL hint to explicitly set parallelism

  • APPEND hint to explicitly set mode

Or the following

ALTER SESSION PARALLEL FORCE to force PARALLEL DML mode

Yes, but requires:

APPEND hint

Yes, but requires:

NOLOGGING attribute set for partition or table


If parallel DML is enabled and there is a PARALLEL hint or PARALLEL attribute set for the table in the data dictionary, then insert operations are parallel and appended, unless a restriction applies. If either the PARALLEL hint or PARALLEL attribute is missing, the insert operation is performed serially. Automatic DOP only parallelizes the DML part of a SQL statement if and only if parallel DML is enabled or forced.

If parallel DML is enabled, then you can use the NOAPPEND hint to perform a parallel conventional insert operation. For example, you can use /*+ noappend parallel */ with the SQL INSERT statement to perform a parallel conventional insert.

SQL> INSERT /*+ NOAPPEND PARALLEL */ INTO sales_hist SELECT * FROM sales;

The advantage of the parallel conventional insert operation is the ability to perform online operations with none of the restrictions of direct-path INSERT. The disadvantage of the parallel conventional insert operation is that this process may be slower than direct-path INSERT.

Parallel DML Tip 2: Direct-Path INSERT

The append mode is the default during a parallel insert operation: data is always inserted into a new block, which is allocated to the table. Therefore, the APPEND hint is optional. You should use append mode to increase the speed of INSERT operations, but not when space utilization must be optimized. You can use NOAPPEND to override append mode.

The APPEND hint applies to both serial and parallel insert operation: even serial insertions are faster if you use this hint. The APPEND hint, however, does require more space and locking overhead.

You can use NOLOGGING with APPEND to make the process even faster. NOLOGGING means that no redo log is generated for the operation. NOLOGGING is never the default; use it when you want to optimize performance. It should not typically be used when recovery is needed for the table or partition. If recovery is needed, be sure to perform a backup immediately after the operation. Use the ALTER TABLE [NO]LOGGING statement to set the appropriate value.

Parallel DML Tip 3: Parallelizing INSERT, MERGE, UPDATE, and DELETE

When the table or partition has the PARALLEL attribute in the data dictionary, that attribute setting is used to determine parallelism of INSERT, UPDATE, and DELETE statements and queries. An explicit PARALLEL hint for a table in a statement overrides the effect of the PARALLEL attribute in the data dictionary.

You can use the NO_PARALLEL hint to override a PARALLEL attribute for the table in the data dictionary. In general, hints take precedence over attributes.

DML operations are considered for parallelization only if the session is in a PARALLEL DML enabled mode. (Use ALTER SESSION ENABLE PARALLEL DML to enter this mode.) The mode does not affect parallelization of queries or of the query portions of a DML statement.

Parallelizing INSERT SELECT

In the INSERT ... SELECT statement, you can specify a PARALLEL hint after the INSERT keyword, in addition to the hint after the SELECT keyword. The PARALLEL hint after the INSERT keyword applies to the INSERT operation only, and the PARALLEL hint after the SELECT keyword applies to the SELECT operation only. Thus, parallelism of the INSERT and SELECT operations are independent of each other. If one operation cannot be performed in parallel, it has no effect on whether the other operation can be performed in parallel.

The ability to parallelize insert operations causes a change in existing behavior if the user has explicitly enabled the session for parallel DML and if the table in question has a PARALLEL attribute set in the data dictionary entry. In that case, existing INSERT SELECT statements that have the select operation parallelized can also have their insert operation parallelized.

If you query multiple tables, you can specify multiple SELECT PARALLEL hints and multiple PARALLEL attributes.

Example 8-11 shows the addition of the new employees who were hired after the acquisition of ACME.

Example 8-11 Parallelizing INSERT SELECT

INSERT /*+ PARALLEL(employees) */ INTO employees
SELECT /*+ PARALLEL(ACME_EMP) */ *  FROM ACME_EMP;

The APPEND keyword is not required in this example because it is implied by the PARALLEL hint.

Parallelizing UPDATE and DELETE

The PARALLEL hint (placed immediately after the UPDATE or DELETE keyword) applies not only to the underlying scan operation, but also to the UPDATE or DELETE operation. Alternatively, you can specify UPDATE or DELETE parallelism in the PARALLEL clause specified in the definition of the table to be modified.

If you have explicitly enabled parallel DML for the session or transaction, UPDATE or DELETE statements that have their query operation parallelized can also have their UPDATE or DELETE operation parallelized. Any subqueries or updatable views in the statement can have their own separate PARALLEL hints or clauses, but these parallel directives do not affect the decision to parallelize the update or delete. If these operations cannot be performed in parallel, it has no effect on whether the UPDATE or DELETE portion can be performed in parallel.

Example 8-12 shows the update operation to give a 10 percent salary raise to all clerks in Dallas.

Example 8-12 Parallelizing UPDATE and DELETE

UPDATE /*+ PARALLEL(employees) */ employees
SET salary=salary * 1.1 WHERE job_id='CLERK' AND department_id IN
  (SELECT department_id FROM DEPARTMENTS WHERE location_id = 'DALLAS');

The PARALLEL hint is applied to the UPDATE operation and to the scan.

Example 8-13 shows the removal of all products of category 39 because that business line was recently spun off into a separate company.

Example 8-13 Parallelizing UPDATE and DELETE

DELETE /*+ PARALLEL(PRODUCTS) */ FROM PRODUCTS 
WHERE category_id = 39;

Again, the parallelism is applied to the scan and UPDATE operations on the table employees.

Incremental Data Loading in Parallel

Parallel DML combined with the updatable join views facility provides an efficient solution for refreshing the tables of a data warehouse system. To refresh tables is to update them with the differential data generated from the OLTP production system.

In the following example, assume a refresh of a table named customers that has columns c_key, c_name, and c_addr. The differential data contains either new rows or rows that have been updated since the last refresh of the data warehouse. In this example, the updated data is shipped from the production system to the data warehouse system by means of ASCII files. These files must be loaded into a temporary table, named diff_customer, before starting the refresh process. You can use SQL*Loader with both the parallel and direct options to efficiently perform this task. You can use the APPEND hint when loading in parallel as well.

After diff_customer is loaded, the refresh process can be started. It can be performed in two phases or by merging in parallel, as demonstrated in the following:

Updating the Table in Parallel

The following statement is a straightforward SQL implementation of the update using subqueries:

UPDATE customers SET(c_name, c_addr) = (SELECT c_name, c_addr
FROM diff_customer WHERE diff_customer.c_key = customer.c_key)
WHERE c_key IN(SELECT c_key FROM diff_customer);

Unfortunately, the two subqueries in this statement affect performance.

An alternative is to rewrite this query using updatable join views. To rewrite the query, you must first add a primary key constraint to the diff_customer table to ensure that the modified columns map to a key-preserved table:

CREATE UNIQUE INDEX diff_pkey_ind ON diff_customer(c_key) PARALLEL NOLOGGING;
ALTER TABLE diff_customer ADD PRIMARY KEY (c_key);

You can then update the customers table with the following SQL statement:

UPDATE /*+ PARALLEL(cust_joinview) */
(SELECT /*+ PARALLEL(customers) PARALLEL(diff_customer) */
CUSTOMER.c_name AS c_name CUSTOMER.c_addr AS c_addr,
diff_customer.c_name AS c_newname, diff_customer.c_addr AS c_newaddr
   FROM diff_customer
   WHERE customers.c_key = diff_customer.c_key) cust_joinview
   SET c_name = c_newname, c_addr = c_newaddr;

The underlying scans feeding the join view cust_joinview are done in parallel. You can then parallelize the update to further improve performance, but only if the customers table is partitioned.

Inserting the New Rows into the Table in Parallel

The last phase of the refresh process consists of inserting the new rows from the diff_customer temporary table to the customers table. Unlike the update case, you cannot avoid having a subquery in the INSERT statement:

INSERT /*+PARALLEL(customers)*/ INTO customers SELECT * FROM diff_customer s);

However, you can guarantee that the subquery is transformed into an anti-hash join by using the HASH_AJ hint. Doing so enables you to use parallel INSERT to execute the preceding statement efficiently. Parallel INSERT is applicable even if the table is not partitioned.

Merging in Parallel

You can combine update and insert operations into one statement, commonly known as a merge. The following statement achieves the same result as all of the statements in "Updating the Table in Parallel" and "Inserting the New Rows into the Table in Parallel":

MERGE INTO customers USING diff_customer
ON (diff_customer.c_key = customer.c_key) WHEN MATCHED THEN
  UPDATE SET (c_name, c_addr) = (SELECT c_name, c_addr 
  FROM diff_customer WHERE diff_customer.c_key = customers.c_key) 
WHEN NOT MATCHED THEN
 INSERT VALUES (diff_customer.c_key,diff_customer.c_data);