My presentation at DOAG 2015

2.  Independent consultant
 Performance Troubleshooting
 In-house workshops
 Cost-Based Optimizer
 Performance By Design
 Oracle ACE Director
 Member of OakTable Network

3. This session explains the new PX operators
introduced with Oracle 12c so that you know
them next time you look at a 12c Parallel
Execution plan
More info / details?
On my blog (tag 12cR1, New Features, Parallel Execution)
You want more?
Ask for my Parallel Execution Masterclass seminar

4.  Three important aspects of Parallel Execution
 Major 12c Parallel Execution new features
 Some more new 12c Parallel Execution features
 Conclusion / Q & A

5.  Oracle Parallel Execution employs Producer
Consumer Model
 Parallel Execution plan organised in so called
Data Flow Operations (DFOs) and DFO Trees
 DFOs are organised in Parent child relations,
the child produces / sends data, the parent
consumes / receives data
 Requires two sets of Parallel Execution Server

10.  Producer Consumer model limitation: Only
one active pair of DFOs per DFO tree allowed
 Depending on plan shape and operations
multiple DFO pairs could be active
 To prevent this Oracle adds artificial blocking
operations that buffer data
=> Increased PGA / TEMP requirements

12.  Producer consumer model means redistribution of
data
 Depending on distribution methods used and
actual data pattern the data might not be
distributed evenly
 Data distribution skew means that some Parallel
Execution Servers need to process more data than
others
=> Not all PX Servers busy all the time

15.  Three important aspects of Parallel Execution
 Major 12c Parallel Execution new features
 Some more new 12c Parallel Execution features
 Conclusion / Q & A

16.  New distribution method HYBRID HASH
 Dynamic switch between BROADCAST and
HASH at execution time
 Can detect and address data distribution skew
(current implementation only limited scope)
 Only partially useful at present but is supposed
to be improved in upcoming releases

21.  New feature CONCURRENT UNION ALL –
executes multiple branches of a UNION ALL
concurrently
 Usually only relevant in case of serial branches
(or mixture of parallel and serial branches)
 New operator PX SELECTOR selects one of the
available slaves to execute serial branch

24.  New distribution method 1 SLAVE
 Supports new plan shapes potentially
collapsing multiple DFO trees into a single one
 Simplifies PX plan generation and avoids odd
side effects of having multiple DFO trees
 But can lead to more BUFFERED operations

27.  New feature parallel FILTER subquery
evaluation
 Again simplifies PX plan shape and allows
filter subquery evaluation in the Parallel
Execution Servers => can reduce runtime
 Prevents certain pre-12c plan shapes leading to
multiple DFO trees
 Not always used, 12c can fall back to old shape

30.  Three important aspects of Parallel Execution
 Major 12c Parallel Execution new features
 Some more new 12c Parallel Execution
features
 Conclusion / Q & A

31.  New feature replicated parallel full table scan –
instead of BROADCAST distribution
 Only applicable to parallel full table scans
eligible for BROADCAST distribution
 Instead of chunked parallel full table scan with
distribution afterwards each slave performs the
complete full scan directly

34.  New feature EXPRESSION EVALUATION
operator
 Can be used for scalar subqueries in the projection
of parallel plans (but also old shape possible
depending on SQL features used and combined)
 Good: Plan shape corresponds now to the “rules”
how to read with scalar subqueries
 Bad: Some odd behaviour at runtime spotted

37.  Many new PX operators and plan shapes
introduced with 12c
 Biggest change in this area since explicit
introduction of PX operators in Oracle 10g
 Not as revolutionary as other new features but
a good sign that Oracle continues innovation in
this area, too

38. Q & A