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Hash Join Buffered
IntroductionA few years ago Jonathan Lewis published a blog post that described one of the interesting side effects of Oracle's Parallel Execution implementation: Sometimes operations that usually are...
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HAVING Cardinality
When performing aggregate GROUP BY operations an additional filter on the aggregates can be applied using the HAVING clause.Usually aggregates are one of the last steps executed before the final result...
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Exadata Smart Scan Projection Limitation
Here is an interesting limitation to Exadata Smart Scans - if more than 254 columns from a table (not HCC compressed, more on that in moment) need to be projected, Smart Scans for that particular...
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QB_NAME hint query block name length limitation
Oracle 10g introduced the QB_NAME hint that can come handy in case hints need to be applied to more complex statements, in particular when possibly multiple layers of views / subqueries are...
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“Cost-free” joins – 1
Recently I came across some interesting edge cases regarding the costing of joins. They all have in common that they result in (at first sight) unexpected execution plans, but only some of them are...
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“Cost-free” joins – 2
In the previous post I've demonstrated an unexpected Nested Loop Join caused by an extreme data distribution. Although unexpected at first sight, the performance of the execution plan selected by the...
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Free Webinar
It's webinar time again.Join me on Wednesday, May 8th at AllThingsOracle.com for an overview session on the specifics of Oracle Parallel Execution.The session starts at 16:00 UK (17:00 Central...
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ASM AU Size And LMT AUTOALLOCATE
When using Locally Managed Tablespaces (LMT) with variable, system managed extent sizes (AUTOALLOCATE) and data files residing in ASM the Allocation Unit (AU) size can make a significant difference to...
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New Version Of XPLAN_ASH Tool – Video Tutorial
A new major release (version 3.0) of my XPLAN_ASH tool is available for download.You can download the latest version here.In addition to many changes to the way the information is presented and many...
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TIMESTAMP WITH TIME ZONE Aggregation
The TIMESTAMP WITH TIME ZONE data type that got introduced a long time ago is known for some oddities, for example Tony Hasler has a nice summary of some of them here.Here is another oddity that shows...
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View Data Volume Estimates
When the optimizer has to estimate the data volume (the BYTES column in the plan output), it usually bases this information on the column statistics, if applicable and available (think of complex...
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New Version Of XPLAN_ASH Utility
A new version of the XPLAN_ASH tool (detailed analysis of a single SQL statement execution) is available for download. The previous post includes links to video tutorials explaining what the tool is...
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Analysing Parallel Execution Skew – Video Tutorial
Along the new mini series "Parallel Execution Skew" at "AllThingsOracle.com" that provides some information what you can do if you happen to have a parallel SQL execution that is affected by work...
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Parallel Execution Overhead
I've started a new mini series about "Parallel Execution Skew" at AllThingsOracle.com. In order to avoid bloating the articles too much there I'll post some accompanying notes here on my blog.If you...
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New Version Of XPLAN_ASH Utility
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...
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Analysing Parallel Execution Skew – Data Flow Operations (DFOs) And DFO Trees
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...
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Analysing Parallel Execution Skew – Without Diagnostics / Tuning Pack License
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...
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Parallel Execution Skew – Demonstrating Skew
This is just a short notice that the next part of the mini-series "Parallel Execution Skew" is published at AllThingsOracle.com Related posts: Analysing Parallel Execution Skew – Video Tutorial...
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12c New Optimizer Features
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...
View Article12c Hybrid Hash Distribution with Skew Detection / Handling
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...
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