Parallel Query in RAC - Expert Oracle RAC 12c - page 353

Query
Coordinator

QC

Consumers P01 P02

P01

P02

Producers

P05 P06

P03

P04

Node 1 Node 2

Figure 12-3. PX execution in RAC

In Figure 12-3 , distribution of row pieces between the PX server processes is shown as the connection between
the PX processes. PQ distribution in an inter-instance parallel operation flows through the private network (cluster
interconnect).

Placement of PX Servers
Placement of PX servers can be controlled using a few techniques in RAC. Traditionally, in earlier versions of Oracle
RAC, a combination of instance_group and parallel_instance_group parameters was employed to control PX server
placement, but from Oracle Database 11g onward, it is preferable to use services to control the placement of PX
servers.

Instance_group and Parallel_instance_group
The parameter instance_group specifies the membership of an instance to an instance group. PX servers are
allocated from an instance group specified by the parallel_instance_group parameter. Adjustment to these two
parameters can be utilized to control the placement of PX servers.
The following discussion uses a three-node cluster configuration with instances named orcl1 , orcl2 , and orcl3 .
In the following parameter setup, the instance_groups parameter is specified at the instance level using the syntax
<instance>.<parameter>. The value of the parameter orcl1.instance_groups is set to 'po','scm' and specifies that
the orcl1 instance is a member of instance groups po and scm . Similarly, the orcl2 instance is a member of the po
and scm instance groups, and the orcl3 instance is a member of the finance and scm instance groups. Hence, all
three instances are members of the scm instance group, instances 1 and 2 are members of the po instance group, and
instance 3 is the sole member of the finance instance group.

orcl1.instance_groups='po','scm'
orcl2.instance_groups='po','scm'
orcl3.instance_groups='finance','scm'

With the parameter setup just discussed, we can alter the parallel_instance_group parameter to control PX server
allocation. In the following example, the value of parameter parallel_instance_group is set to scm in orcl1 instance.