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Parallel Execution Skew - Summary

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 on the topic of Parallel Execution Skew.

Anyone regularly using Parallel Execution and/or relying on Parallel Execution for important, time critical processing should know this stuff. In my experience however almost no-one does, and therefore misses possibly huge opportunities for optimizing Parallel Execution performance.

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 content:

Parallel Execution Skew - Addressing Skew Using Manual Rewrites

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 probably not so commonly known technique of redistributing popular values using an additional re-mapping table.

12c Hybrid Hash Distribution with Skew Detection / Handling - Failing

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 with that new feature, here's a three table join that actually makes use of the feature for one join, only to suffer from the skew problem in the next join that uses the same join expression, but doesn't qualify (yet) for the skew handling feature:

12c 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 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" at "AllThingsOracle.com", so if all you want is a high-level overview I recommend reading the article there.The purpose of this note here is to provide a few more internals and details about that feature.First, just a short summary of the prerequisites of the feature to work:1.

Thinking Clearly About Performance, revised to include Skew

I’ve just updated the “Thinking Clearly” paper to include an absolutely vital section that was, regrettably, missing from the first revision. It’s a section on the subject of skew.

I hope you enjoy.