Parallel Query Tuning - Expert Oracle RAC Performance Diagnostics and Tuning - page 245

• Intra-operation parallelism is the parallelization of an individual operation, where the same

operation is performed on smaller sets of rows by parallel execution servers.

• Inter-operation parallelism happens when two operations run concurrently on different sets

of parallel execution servers with data flowing from one operation into the other.

The query (illustrated in Figure 8-5 ) is parallelized at two stages: the scan phase, also called the Producers
(P0 and P1 operations illustrated in Figure 8-4 ), and the sort phase of the operation, also called the Consumers, with
DOP of four resulting in eight parallel execution servers. This is because it is an inter-operation parallelism where a
parent and child operator can be performed at the same time. As illustrated in Figure 8-5 , each server in the producer
set has a connection to each server in the consumer set.

PRODUCTS
Table

CONSUMERS PRODUCERS

Parallel Execution
servers for ORDER or
GROUP BY operations

Parallel Execution
servers for full
table scan

Table divided into
logical units
called granules

A-G

H-M

QC

Query
Coordinator

N-S

T-Z

Inter-operation
Parallelism

Intra-operation
Parallelism

Intra-operation
Parallelism

Figure 8-5. Intra- and inter-operation parallel processing

The producers who access the data divide the workload among the various parallel execution servers
(determined by the DOP) to retrieve data from the tables. The amount of data sets each parallel execution server will
retrieve is determined by the QC process and is in units called granules. Granules are the smallest unit in which data
is retrieved and managed in the memory structures of Oracle . If the table or table partition being accessed is further
dynamically partitioned into logical units (also called granules), these consist of a range of data blocks accessed by
the PX processes. To get an even distribution of the work among the various PX processes, the number of granules is
normally much higher than the requested DOP.