Geeky PL/SQL tracer/profiler : Another step

This is my second post under the theme of how to extend our capabilities to trace and profile PL/SQL code.This time motivated by a comment from Luca Canali on my previous post  :

So based on my previous work on geeky PL/SQL tracer let’s see how we can obtain a geeky PL/SQL on-CPU Flame Graph !

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Geeky PL/SQL tracer/profiler : First step

This blog post is about how to extend our capabilities to trace and profile PL/SQL code.It’s primarily motivated by few tweets from Franck Pachot and of course because it’s FUN !

Capture 02

Capture 01

So in the first part of this series we are going to answer to this questions : Can we map those underling function to the source PL/SQL object and line number ? Can we obtain a full trace ? Of course yes otherwise there will be no blog post :p

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Getting Latch holder info out of the state objects in SGA memory

Updated 12/05/2017

It will be great to have a tool that will extract latch holder information directly from state objects  stored inside the SGA. This way we will reduce the overhead when troubleshooting latch contention an beside that it’s also cool !!

This may sound difficult ! How to proceed ? and the answer is …. Memory reference tracing !

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Since oracle Adaptive Dynamic Sampling  result are stored inside SPD as a special directive type “DYNAMIC_SAMPLING_RESULT”. This will allow the result of ADS to be persisted on disk,thus it will survive memory flush and database restart.For more detailed info please check Mauro Pagano blog Post Something new about SQL Plan Directives and 12.2

We may ask at this moment is there some sanity check that will trigger the refresh of this ADS result stored as sql plan directive when the table data is marked stale (STALE_PERCENT) ?

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Run-time side effect of dropping a composite unused index

Unused Index although they seem not needed , dropping them must be carefully tested  as it can have some side effect. As explained by Jonathan Lewis, due to index sanity check cardinality miss estimate can happen after dropping the unused index.

“The reason for this is the “index sanity check”. When the optimizer is doing its cardinality estimates, if it see equality conditions on the set of columns that make up an index it can use the distinct_keys statistic from the index in the calculation rather than using the standard calculation of multiplying together the num_distinct of the separate columns. ”

There is also a good example of the impact this can have on the query execution plan(switching join method from hash join to nested loop due to the drop of the estimated number of rows returned).Link

In this post i will show another interesting case when this can have an impact on query performance.As Franck Pachot Said :

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