Introduction to parallel executing

Parallel Execution, mainly used in OLAP systems to improve execution efficiency. The execution mechanism of PX is to create a process such as QC when initiating a parallel operation to coordinate information transmission of the parallel server process, the parallel coordination process divides the dataset to be operated into several units according to the number of parallel executions, and distributes the data units to the parallel server process for processing. After the processing, the result set is passed to the parallel coordination process, the parallel coordination process sends the result set to the user.

Number of Parallel Server Processes

When oracle is started, the number of processes will be created based on the parallel_min_servers parameter in the parameter file. When the number of parallel server processes is insufficient, a new parallel server process will be created. The maximum number of allocated processes is parallel_max_servers. The number of parallel server processes used by a statement depends on the operations performed by the statement. If you want to sort the statements after the query, You need to multiply the number by 2, and so on.

A serious wait event "px deq credit: Send blkd" appears in a system. In ID 271767.1, this event is sometimes called a normal wait event, you need to analyze the data according to the actual situation.

This is considered as idle wait event. You shold investigate the sender (decode the senderid). In same cases it is normal that we see high waits on "px deq credit: Send blkd ".

An example: we do a select from a large table in parallel.
select /*+ parallel(sales, 10) +/ * from sales)
Than you see a lot of waits for "px deq credit: Send blkd" in the slave traces. all slaves wait for the QC to get back the credit bit. the QC can not dequeue fast enough the rows from the slaves, beause there are to receive slaves that send rows back to the QC. in this case it is normal.

Parallel query wait events (Doc ID 191103.1)

Waitevent: "px deq credit: Send blkd" (Document ID 271767.1)