本文详细介绍了数据库参数 PARALLEL_EXECUTION_MESSAGE_SIZE 的作用及其对并行执行的影响。该参数控制并行执行的消息大小，通常默认为 2148 字节（当 PARALLEL_AUTOMATIC_TUNING 设置为 false 时）或 4096 字节（当 PARALLEL_AUTOMATIC_TUNING 设置为 true 时）。增大此值可以提升并行恢复等操作的性能，但会增加共享池内存使用。
PARALLEL_EXECUTION_MESSAGE_SIZE specifies the size of messages for parallel execution (formerly referred to as parallel query, PDML, Parallel Recovery, replication).
On most platforms, the default value is 2148bytes if PARALLEL_AUTOMATIC_TUNING is set to false, and 4096 bytes if PARALLEL_AUTOMATIC_TUNING is set to true. The default value is adequate for most applications. Larger values require a larger shared pool. Larger values result in better performance at the cost of higher memory use. For this reason, replication gets no benefit from increasing the size.
Set PARALLEL_EXECUTION_MESSAGE_SIZE = 4096
When using parallel media recovery or parallel standby recovery, increasing the PARALLEL_EXECUTION_MESSAGE_SIZE database parameter to 4K (4096) can improve parallel recovery by as much as 20 percent. Set this parameter on both the primary and standby databases in preparation for switchover operations. Increasing this parameter requires more memory from the shared pool by each parallel execution slave process.
The PARALLEL_EXECUTION_MESSAGE_SIZE parameter is also used by parallel query operations and should be tested with any parallel query operations to ensure there is sufficient memory on the system. A large number of parallel query slaves on a 32-bit installation may reach memory limits and prohibit increasing the PARALLEL_EXECUTION_MESSAGE_SIZE from the default 2K (2048) to 4K.
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/21993926/viewspace-723494/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/21993926/viewspace-723494/
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