|Oracle® Database VLDB and Partitioning Guide
11g Release 2 (11.2)
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This section contains some ideas for improving performance in a parallel execution environment and includes the following topics:
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
SELECT or direct-path
INSERT to store the result set in the database. At a later time, users can view the result set serially.
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.
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. Note that 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
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 Oracle Database Data Warehousing Guide, for more information about bottlenecks.
PLAN statement to see the execution plans for parallel queries. The
PLAN output shows optimizer information in the
CARDINALITY columns. You can also use the
utlxplp.sql script to present the
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
The key points when using
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. Note that the optimizer considers the
BROADCAST distribution method only if the
OPTIMIZER_FEATURES_ENABLE is set to 9.0.2 or higher. See "V$PQ_TQSTAT" for more information.
The following example illustrates how the optimizer intends to execute a parallel query:
explain plan for SELECT /*+ PARALLEL */ cust_first_name, cust_last_name FROM customers c, sales s WHERE c.cust_id = s.cust_id; ---------------------------------------------------------- | Id | Operation | Name | ---------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | PX COORDINATOR | | | 2 | PX SEND QC (RANDOM) | :TQ10000 | | 3 | NESTED LOOPS | | | 4 | PX BLOCK ITERATOR | | | 5 | TABLE ACCESS FULL | CUSTOMERS | | 6 | PARTITION RANGE ALL | | | 7 | BITMAP CONVERSION TO ROWIDS| | | 8 | BITMAP INDEX SINGLE VALUE | SALES_CUST_BIX | ---------------------------------------------------------- Note ----- - Computed Degree of Parallelism is 2 - Degree of Parallelism of 2 is derived from scan of object SH.CUSTOMERS
When you want to refresh your data warehouse database using parallel insert, update, or delete operations on a data warehouse, there are additional issues to consider when designing the physical database. These considerations do not affect parallel execution operations. These issues are:
If a parallel restriction is violated, the operation is simply performed serially. If a direct-path
INSERT restriction is violated, then the
APPEND hint is ignored and a conventional insert operation is performed. No error message is returned.
For tables that do not have the parallel DML
itl invariant property (tables created before Oracle Database release 9.2 or tables that were created with the
COMPATIBLE initialization parameter set to less than
9.2), the degree of parallelism (DOP) equals the number of partitions or subpartitions. That means that, if the table is not partitioned, the query runs serially. To see what tables do not have this property, issue the following statement:
SELECT u.name, o.name FROM obj$ 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.
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
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.
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.
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 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.
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.
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';
[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
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.
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
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
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
NOARCHIVELOG, then all operations that can be done without logging do not generate logs, regardless of the specified logging attribute.
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
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
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
PARALLEL clause in the
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
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
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
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
NOVALIDATE state. In the following example, the
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;
This section provides an overview of parallel DML functionality. The topics covered include:
The functionality available using an
INSERT statement can be summarized as shown in Table 8-5:
See text in this section for information about using the
Yes, but requires
and one of the following:
Or the following
Yes, but requires:
Yes, but requires:
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. Note that 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
*/ 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
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.
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
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
[NO]LOGGING statement to set the appropriate value.
When the table or partition has the
PARALLEL attribute in the data dictionary, that attribute setting is used to determine parallelism of
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
DML enabled mode. (Use
DML to enter this mode.) The mode does not affect parallelization of queries or of the query portions of a DML statement.
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
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
SELECT statements that have the select operation parallelized can also have their insert operation parallelized.
If you query multiple tables, you can specify multiple
PARALLEL hints and multiple
Example 8-10 shows the addition of the new employees who were hired after the acquisition of
PARALLEL hint (placed immediately after the
DELETE keyword) applies not only to the underlying scan operation, but also to the
DELETE operation. Alternatively, you can specify
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,
DELETE statements that have their query operation parallelized can also have their
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
DELETE portion can be performed in parallel.
Example 8-11 shows the update operation to give a 10 percent salary raise to all clerks in Dallas.
UPDATE /*+ PARALLEL(employees) */ employees SET SAL=SAL * 1.1 WHERE JOB='CLERK' AND DEPTNO IN (SELECT DEPTNO FROM DEPT WHERE LOCATION='DALLAS');
PARALLEL hint is applied to the
UPDATE operation and to the scan.
Example 8-12 shows the removal of all products in the grocery category because the grocery business line was recently spun off into a separate company.
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
customer that has columns
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.
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:
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.
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 /*+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.
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);