Database Reference
In-Depth Information
Query
Coordinator
QC
Consumers
P01
P03
P02
P04
Producers
P05
P07
P06
P08
Figure 12-2. PX execution in single instance
Also, in a single-instance database, the PX server processes communicate by exchanging PX buffers.
PX Execution in RAC
PX execution in RAC follows the same principle as a single-instance PX execution, except that PX server processes can
be allocated from any and all active instances. 1 Allocation of PX server processes from multiple instances to execute
a SQL statement is known as inter-instance parallelism, and these PX server processes exchange message buffers
between them through private networks. Allocation of PX server processes within the local instance is termed as
intra-instance parallelism.
As we saw in Listing 12-1, row pieces (message buffers in network terminology) are transmitted between PX
server sets using the PX Distribution method of HASH and BROADCAST (there are few other Distribution methods,
namely, Partition , none, etc.). In RAC, row pieces are distributed through the private network. The optimal placement
of PX server processes is of paramount importance to reduce private network traffic generated by PX workload.
RAC databases with higher PX workloads generally have higher bandwidth requirements in the private
interconnect. Oracle Database versions 11g/12c are optimized to minimize private network traffic with optimal
localization of PX server processes.
Figure 12-3 shows a PX server allocation scheme in RAC. In this figure, PX server processes are allocated in both
nodes 1 and 2 to perform parallel execution. Two PX servers on each node act as consumers, and two PX servers on
each node act as producers. Producer PX servers distribute row pieces to consumer PX servers through the private
interconnect. In this example, to minimize interconnect traffic, both producers and their consumers should be
allocated from the same node.
1 I will discuss methods to control PX server allocation to an instance or a subset of instance later in this chapter.
 
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