基于并行查询要启动并行进程、分配任务与系统资源、合并结果集,这些都是比较消耗资源, 但我们为能够减少执行事务的时间使用parallel HINT还是值得的,尤其在ODS系统中报表统计等方面更有使用意义. 一般而言主要在如下情况使用parallel HINT 1.表的数据量很大,超过一千万; 2.数据库主机是多个CPU; 3.系统的当前负载较低;...
The plan indicates that only one partition needs to be queried. So, the system did not run a parallel query. To run a parallel query on a single partition, you can only add thePARALLELhint to the query. Then, you can execute theEXPLAINstatement to view the execution plan generated by th...
② 语句级别,SQL语句中有Hint提示,例如,使用PARALLEL或者PARALLEL_INDEX。如,“SELECT /*+ PARALLEL(T 4) */ FROM T;”。 ③ SQL语句中引用的对象被设置了并行属性。在表和索引的定义中增加并行度属性,该属性可以在创建表和索引时设置,也可对已创建的表和索引的并行度属性进行修改。例如,“ALTER TABLE TB_NA...
(See "Specifying a Query Block in a Hint", tablespec::=) The PARALLEL hint instructs the optimizer to use the specified number of concurrent servers for a parallel operation. This hint overrides the value of the PARALLEL_DEGREE_POLI...
当PARALLEL_ADAPTIVE_MULTI_USER参数设置为TRUE,启用设计的适当算法,在多用户环境下使用并行执行提升性能。这个算法基于查询开始时的系统负载自动减少请求的并行度。有效的并行度是基于默认的并行度,或者来自表或HINT的并行度,通过减少系数进行分割。 The algorithm assumes that the system has been tuned for optimal pe...
If the distribution or location of data changes, Oracle automatically adapts to optimize the parallelization for each execution of the query portion of a SQL statement. Parallel scans by block range break the table or index into pieces delimited by high and low ROWID values. The table or index...
TABLOCK hint provides lock escalation on the table level for source tables. Table level exclusive locks reduce concurrency, which means another sessions cannot insert or update a table when the parallel insert is running In previous query create parallel execution plan. Left to right it contains: ...
When set to HASH, this parameter converts a correlated EXISTS subquery into a view query block and semi-join which is evaluated in parallel. For a specific query, place the HASH_SJ or MERGE_SJ hint into the EXISTS subquery. HASH_SJ uses a hash semi-join and MERGE_SJ uses a sort merge...
Oracle first retrieves the DEGREE and INSTANCES specifications from the definition of the target table. If the statement contains a parallel hint associated with the UPDATE or DELETE, the hint overrides the specification in the table definition. ...
If the execution involves a full table scan, the statement must contain a PARALLEL hint specifying the corresponding table, or the corresponding table must have a parallel declaration in its definition. If the execution involves an index range scan spanning multiple partitions, the statement must con...