Database Reference
In-Depth Information
•
One Slave
: Producers send all rows to a single consumer. Available from version 12.1
onward only.
•
QC Random
: Each producer sends all rows to the query coordinator. The order isn't important
(hence, random). This is the most commonly used distribution to communicate with the
query coordinator.
•
QC Order
: Each producer sends all rows to the query coordinator. The order is important.
For example, this is used by a sort executed in parallel to send data to the query coordinator.
reLatIONShIp BetWeeN paraLLeL OperatIONS
The following relationships between parallel operations are used in execution plans executed in parallel:
•
parallel to serial (
P->S
): a parallel operation sends data to a serial operation. For example, this is
used in every execution plan to send data to the query coordinator.
•
parallel to parallel (
P->P
): a parallel operation sends data to another parallel operation. This is
used when there are two sets of slave processes.
•
parallel combined with parent (
PCWP
): an operation is executed in parallel by the same
slave processes that also execute the parent operation in the execution plan. Therefore, no
communication takes place.
•
parallel combined with child (
PCWC
): an operation is executed in parallel by the same
slave processes that also execute the child operation in the execution plan. Therefore, no
communication takes place.
•
Serial to parallel (
S->P
): a serial operation sends data to a parallel operation. Because most of
the time this is inefficient, it should be avoided. There is one main reason for inefficiency: a single
process might not be able to produce data as fast as several processes can consume it. If that's
the case, the consumers spend much of their time waiting for data instead of doing real work.
•
Serial combined with parent (SCWp): an operation is executed serially by the same slave process
that also executes the parent operation in the execution plan. Therefore, no communication takes
place. available from version 12.1 onward.
•
Serial combine with child (SCWC): an operation is executed serially by the same slave process
that also executes the child operation in the execution plan. Therefore, no communication takes
place. available from version 12.1 onward.
In the output generated by the
dbms_xplan
package, the relationship between parallel operations is provided in
the
IN-OUT
column.
Several parallel operations carried out as a sequence are collectively called
data flow operation
(DFO). In many
situations, execution plans have a single data flow operation. However, there are cases where several data flow
operations are required. To know the number of data flow operations, in the output generated by the
dbms_xplan
