12cR1

randolf.geist's picture

12c Parallel Execution New Features: Concurrent UNION ALL - Part 2

In the first part of this series I've focused on the parallel degree chosen by the optimizer when dealing with the new concurrent UNION ALL feature.I've shown that for the variant with serial branches only in the UNION ALL in principle the number of branches dictates the parallel degree determined, even in cases of more complex plans that mix such a serial branch only UNION ALL operator with some other parallel stuff for example via a join.In this part I'll focus on the runtime behaviour of the feature, but before doing so let me show you what happens if you start mixing serial and parallel branches in the UNION ALL, like that (using the identical table setup as in the previous part):


select count(*) from (
randolf.geist's picture

12c Parallel Execution New Features: Concurrent UNION ALL - Part 1

12c introduces the concurrent UNION ALL operator that allows multiple branches below the UNION ALL to become active concurrently - which is a significant change. Before the introduction of this feature Oracle never executed multiple branches of an execution plan concurrently (in terms of Parallel Execution) - the parallelism so far was about executing the same operations of a particular branch of the execution plan by multiple processes / sessions.

randolf.geist's picture

12c Parallel Execution New Features: Hybrid Hash Distribution - Part 2

In the second part of this post (go to part 1) I want to focus on the hybrid distribution for skewed join expressions.

2. Hybrid Distribution For Skewed Join Expressions

The HYBRID HASH distribution allows to some degree addressing data distribution skew in case of HASH distributions, which I've described in detail already in the past.

randolf.geist's picture

12c Parallel Execution New Features: Hybrid Hash Distribution - Part 1

In this blog post I want to cover some aspects of the the new HYBRID HASH adaptive distribution method that I haven't covered yet in my other posts.As far as I know it serves two purposes for parallel HASH and MERGE JOINs, adaptive broadcast distribution and hybrid distribution for skewed join expressions. In the first part of this post I want to focus on former one (goto part 2).

1. Adaptive Broadcast Distribution For Small Left Row Sources

It allows the PX SEND / RECEIVE operation for the left (smaller estimated row source) of the hash join to decide dynamically at runtime, actually at each execution, if it should use either a BROADCAST or HASH distribution, and correspondingly for the other row source to use then either a ROUND-ROBIN or a HASH distribution, too.

randolf.geist's picture

Parallel Execution 12c New Features Overview

Oracle 12c is the first release since a couple of years that adds significant new functionality in the area of Parallel Execution operators, plan shapes and runtime features. Although 11gR2 added the new Auto DOP feature along with In-Memory Parallel Execution and Statement Queueing, the 12c features are more significant because they introduce new operators that can change both execution plan shape and runtime behaviour.

Here is a list of new features that are worth to note (and not necessarily mentioned in the official documentation and white papers by Oracle):

- The new HYBRID HASH adaptive distribution method, that serves two purposes for parallel HASH and MERGE JOINs:

Chris Antognini's picture

Restrict the query optimizer through OPTIMIZER_FEATURES_ENABLE and Top-Frequency/Hybrid Histograms

To make upgrades easier, I regularly see people considering disabling query optimizer features by setting the OPTIMIZER_FEATURES_ENABLE initialization parameter to a non-default value. My general opinion about this “habit” is summarized in TOP with the following two sentences:

Changing the default value of the OPTIMIZER_FEATURES_ENABLE initialization parameter is only a short-term workaround. Sooner or later the application should be adapted (optimized) for the new database version.

randolf.geist's picture

New Version Of XPLAN_ASH Utility - In-Memory Support

A new version 4.21 of the XPLAN_ASH utility is available for download. I publish this version because it will be used in the recent video tutorials explaining the Active Session History functionality of the script.

As usual the latest version can be downloaded here.

This is mainly a maintenance release that fixes some incompatibilities of the 4.2 version with less recent versions (10.2 and 11.2.0.1).

As an extra however, this version now differentiates between general CPU usage and in-memory CPU usage (similar to 12.1.0.2 Real-Time SQL Monitoring). This is not done in all possible sections of the output yet, but the most important ones are already covered.

randolf.geist's picture

"SELECT * FROM TABLE" Runs Out Of TEMP Space

Now that I've shown in the previous post in general that sometimes Parallel Execution plans might end up with unnecessary BUFFER SORT operations, let's have a look what particular side effects are possible due to this.

What would you say if someone tells you that (s)he just did a simple, straightforward "SELECT * FROM TABLE" that took several minutes to execute without returning, only to then error out with "ORA-01652 unable to extend temp segment", and the TABLE in question is actually nothing but a simple, partitioned heap table, so no special tricks, no views, synonyms, VPD etc. involved, it's really just a plain simple table?

randolf.geist's picture

Unnecessary BUFFER SORT Operations - Parallel Concatenation Transformation

When using Parallel Execution, depending on the plan shape and the operations used, Oracle sometimes needs to turn non-blocking operations into blocking operations, which means in this case that the row source no longer passes its output data directly to the parent operation but buffers some data temporarily in PGA memory / TEMP. This is either accomplished via the special HASH JOIN BUFFERED operation, or simply by adding BUFFER SORT operations to the plan.The reason for such a behaviour in parallel plans is the limitation of Oracle Parallel Execution that allows only a single data redistribution to be active concurrently.

randolf.geist's picture

New Version Of XPLAN_ASH Utility

A new version 4.2 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

There were no too significant changes in this release, mainly some new sections related to I/O figures were added.

One thing to note is that some of the sections in recent releases may require a linesize larger than 700, so the script's settings have been changed to 800. If you use corresponding settings for CMD.EXE under Windows for example you might have to adjust accordingly to prevent ugly line wrapping.

Here are the notes from the change log:

- New sections "Concurrent activity I/O Summary based on ASH" and "Concurrent activity I/O Summary per Instance based on ASH" to see the I/O activity summary for concurrent activity

To prevent automated spam submissions leave this field empty.
Syndicate content