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...)