The next part of the video tutorial explaining the XPLAN_ASH Active Session History functionality has been published. In this part I begin the actual walk-through of the script output.
More parts to follow.
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 184.108.40.206).
As an extra however, (more...)
It's webinar time again.
Join me on Wednesday, January 28th at AllThingsOracle.com
for a session based on a real world customer experience.
The session starts at 3pm UK (16:00 Central European) time. The webinar is totally free and the recording will made available afterwards.
Here's the link to the official landing page
where you can register and below is the official abstract:
After a short introduction into what the Oracle Exadata Database Machine (more...)
I finally got around preparing another part of the XPLAN_ASH video tutorial.
This part is about the main funcationality of XPLAN_ASH: SQL statement execution analysis using Active Session History and Real-Time SQL Monitoring.
In this video tutorial I'll explain what the output of XPLAN_ASH is supposed to mean when using the Active Session History functionality of the script. Before diving into the details of the script output using sample reports I provide some overview and (more...)
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", (more...)
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 (more...)
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 (more...)
There are at least three different ways how the Oracle optimizer can come up with a so called TEMP table transformation
, that is materializing an intermediate result set:- As part of a Star Schema
transformation the repeated access to dimensions can be materialized- As part of evaluating GROUPING SETs
intermediate result sets can be materialized- Common Subquery/Table Expressions (CTE, WITH clause)Probably the most common usage of the materialization is in (more...)
I've published the final part
of my video tutorial and the final part
of my mini series "Parallel Execution Skew" at AllThingsOracle.com
concluding what I planned to publish about the topic of Parallel Execution Skew.
Since all this was published over a longer period of time this post therefore is a summary with pointers to the material.
If you want to get an idea what the material is about, the following video summarizes the (more...)
A new version 4.1 of the XPLAN_ASH utility is available for download.As usual the latest version can be downloaded here.
This version in particular supports now the new 12c "Adaptive" plan feature - previous versions don't cope very well with those if you don't add the "ADAPTIVE" formatting option manually.
Here are the notes from the change log:
- GV$SQL_MONITOR and GV$SQL_PLAN_MONITOR can now be customized in the
settings as table names in (more...)
This is just a short note that the next part of the mini series
about Parallel Execution skew has been published at AllThingsOracle.com
After having shown in the previous instalment
of the series that Oracle 12c added a new feature that can deal with Parallel Execution skew (at present in a limited number of scenarios) I now demonstrate in that part how the problem can be addressed using manual query rewrites, in particular the (more...)
This is just an addendum to the previous post
demonstrating one example (out of many possible) where the join skew handling feature fails. The test case setup is the same as in the previous post.As mentioned in the AllThingsOracle.com article
and in the introduction of the previous post, the feature at present only applies to a rather limited number of scenarios. To wrap things up and to give an idea what can happen (more...)
Besides the officially available information about new optimizer features in 12c it is always a good idea to have a look at the internal optimizer parameters that show what features are enabled when running with OPTIMIZER_FEATURES_ENABLE = 220.127.116.11. Here is the list of internal optimizer parameters and fix controls that are different between 18.104.22.168
_optimizer_partial_join_eval partial join evaluation parameter
_optimizer_unnest_scalar_sq enables (more...)
Oracle 12c introduces several new features in the area of Parallel Execution. Over the next couple of weeks I attempt to publish more about them - Jonathan Lewis for example already published a note
about the new "PQ Replication" feature that applies to the BROADCAST distribution of small tables.One important new feature is the automatic skew handling
for parallel joins. I've already given an overview of the feature in my mini-series "Parallel Execution Skew" (more...)
This is just a short notice that the next part of the mini-series
"Parallel Execution Skew" is published at AllThingsOracle.com
This is the third part of the video tutorial "Analysing Parallel Execution Skew". In this part I show how to analyse a parallel SQL execution regarding Parallel Execution Skew.
If you don't have a Diagnostics / Tuning Pack license the options you have for doing that are quite limited, and the approach, as demonstrated in the tutorial, has several limitations and shortcomings.
Here is the video:
If you want to reproduce or play around with (more...)
This is the second part of the video tutorial "Analysing Parallel Execution Skew". In this part I introduce the concept of "Data Flow Operations (DFOs)" and "DFO Trees", which is what a Parallel Execution plan is made of. DFOs / DFO Trees are specific to Parallel Execution and don't have any counterpart in a serial execution plan.
Understanding the implications of DFOs / DFO Trees is important as prerequisite for understanding some of the effects (more...)
A minor update 4.01 to the XPLAN_ASH utility is available for download.As usual the latest version can be downloaded here.
These are the notes from the change log:
- More info for RAC Cross Instance Parallel Execution: Many sections now show a GLOBAL aggregate info in addition to instance-specific data
- The Parallel Execution Server Set detection and ASSUMED_DEGREE info now makes use of the undocumented PX_STEP_ID and PX_STEPS_ARG info (bit mask part (more...)
I've started a new mini series about "Parallel Execution Skew"
. In order to avoid bloating the articles too much there I'll post some accompanying notes here on my blog.If you follow the initial post
you'll see that the sample query demonstrated there scales almost perfectly with Parallel Execution - the serial execution
takes 57 seconds
on my test system, whereas the a Parallel Execution at a DOP of 4
takes something (more...)
Along the new mini series "Parallel Execution Skew"
that provides some information what you can do if you happen to have a parallel SQL execution that is affected by work distribution problems I'm publishing a series of video tutorials that explain how you can actually detect and measure whether a SQL execution is affected by skew or not.
Originally this tutorial was planned as one part (Part 5 actually) of the XPLAN_ASH (more...)