Adaptive Cursor Sharing Fail

Here is another example (besides the fact that Adaptive Cursor Sharing only gets evaluated during a PARSE call (still valid in 12c) and supports a maximum of 14 bind variables) I've recently come across at a client site where the default implementation of Adaptive Cursor Sharing fails to create a more suitable execution plan for different bind variable values.Broken down to a bare minimum the query was sometimes executed using non-existing values for a (more...)

Nested Loop Join Physical I/O Optimizations

Having done my mini-series on Nested Loop join logical I/O optimizations a while ago I unfortunately never managed to publish anything regarding the Nested Loop join physical I/O optimizations, which are certainly much more relevant to real-life performance.Therefore the main purpose of this blog post is to point you to Nikolay Savvinov's (whose blog I can recommend in general) great mini-series covering various aspects of these optimizations:Part 1Part 2Part 3Summary (more...)

Oracle Database Cloud (DBaaS) Performance Consistency – Part 2

This is the second part of this installment, comparing the performance consistency of the DBaaS cloud offering with a dedicated physical host. This time instead of burning CPU using a trivial PL/SQL loop (see part 1) the test harness executes a SQL statement that performs logical I/O only, so no physical I/O involved.

In order to achieve that a variation of Jonathan Lewis' good old "kill_cpu" script got executed. In principle each thread performed (more...)

DBMS_STATS – Gather statistics on tables with many columns – 12c update

This is just a short 12c update on my post about gathering statistics on tables with many columns from some time ago.I'm currently investigating the "Incremental Statistics" feature in 12.1.0.2 for a client, which probably will be worth one or more other posts, but since we're still in the process of evaluating and installing various patches it's too early to write about that.As part of the investigation I've noticed a (more...)

Video Tutorial: XPLAN_ASH Active Session History – Part 9

The next part of the video tutorial explaining the XPLAN_ASH Active Session History functionality continuing the actual walk-through of the script output.

More parts to follow.

Video Tutorial: XPLAN_ASH Active Session History – Part 8

The next part of the video tutorial explaining the XPLAN_ASH Active Session History functionality continuing the actual walk-through of the script output.

More parts to follow.


Comparing Columns Containing NULL Values

Prompted by a (not really that) recent discussion on the OTN forum I've decided to publish this note.Sometimes you have the task of comparing column values and handling the NULL value cases correctly makes this rather cumbersome for columns that are allowed to be NULL.The "official" SQL way of comparing two column values and to find out whether they are equal or not - under the assumption that having NULL in both columns (more...)

New Version Of XPLAN_ASH Utility

A new version 4.23 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

This version comes only with minor changes, see the change log below.

Here are the notes from the change log:

- Finally corrected the very old and wrong description of "wait times" in the script comments, where it was talking about "in-flight" wait events but that is not correct. ASH performs a "fix-up" (more...)

Combining Features – Wrong Results With Scalar Subquery Caching

Quite often you can get into trouble with Oracle when you start combining different features.In this case of one my clients it is the combination of user-defined PL/SQL functions that can raise exceptions (think of currency conversion and a non-existent currency code gets passed into the function), DML error logging and attempting to improve performance by wrapping the PL/SQL function call into a scalar subquery to benefit from the built-in scalar subquery caching feature (more...)

Oracle Database Cloud (DBaaS) Performance Consistency

As Oracle ACE Director I got an extended trial license for Oracle's Cloud offerings, in particular the "Database as a Service" offering. As part of the (ongoing) evaluation I try to get an idea how consistent the performance of such an service is, which might be one of the concerns one might have when considering cloud offerings in general.

For my tests I've set up a 11.2.0.4 single instance database using "4 (more...)

SOUG Training Day May 2016 in Switzerland

I will be one of the speakers at the first SOUG performance training day, together with Christian Antognini, Franck Pachot and Clemens Bleile.

The event will take place in May this year in Switzerland at two different locations / days (one in German language, one in French, except mine, which will be in English).

My presentations will be:

- Analyzing and Troubleshooting Oracle Parallel Execution

- Advanced Oracle Troubleshooting

Hope to see you there!

Below (more...)

Advanced Oracle Troubleshooting – One Day Seminar, Moscow

I got invited by Luxoft Training to deliver my one day seminar "Advanced Oracle Troubleshooting" in Moscow end of March.

More details about what I cover in this seminar can be found here.

If you're interested, Luxoft Training has set up a page in Russian where you can find more details - but note that the seminar language will be English.

Big Nodes, Concurrent Parallel Execution And High System/Kernel Time

This probably only is relevant for customers that run Oracle on big servers with lots of cores, like some of my clients that make use of the Exadata Xn-8 servers, like a X4-8 with 120 cores / 240 CPUs per node.

They recently came up with a re-write of a core application functionality. Part of this code did start the same code path for different data sets potentially several times concurrently ending up with many (more...)

DML Operations On Partitioned Tables Can Restart On Invalidation

It's probably not that well known that Oracle can actually rollback / re-start the execution of a DML statement should the cursor become invalidated. By rollback / re-start I mean that Oracle actually performs a statement level rollback (so any modification already performed by that statement until that point gets rolled back), performs another optimization phase of the statement on re-start (due to the invalidation) and begins the execution of the statement from scratch. Note (more...)

Video Tutorial: XPLAN_ASH Active Session History – Part 7

The next part of the video tutorial explaining the XPLAN_ASH Active Session History functionality continuing the actual walk-through of the script output.

More parts to follow.

12c Parallel Execution New Features: Parallel FILTER Subquery Evaluation – Part 3: The Optimizer And Distribution Methods

As mentioned in the first and second part of this instalment the different available distribution methods of the new parallel FILTER are selected automatically by the optimizer - in this last post of this series I want to focus on that optimizer behaviour.It looks like there are two new optimizer related parameters that control the behaviour of the new feature: "_px_filter_parallelized" is the overall switch to enable/disable the new parallel filter capability - and (more...)

New Version Of XPLAN_ASH Utility

A new version 4.22 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

This version primarily addresses an issue with 12c - if the HIST mode got used to pull ASH information from AWR in 12c it turned out that Oracle forgot to add the new "DELTA_READ_MEM_BYTES" columns to DBA_HIST_ACTIVE_SESS_HISTORY - although it got officially added to V$ACTIVE_SESSION_HISTORY in 12c. So now I had to implement (more...)

IT Tage 2015 – "Analysing and troubleshooting Parallel Execution" presentation material

Thanks to all attending my presentation "Analysing and troubleshooting Parallel Execution" at the IT Tage conference 2015 in Frankfurt, Germany. You can download the presentation material here in Powerpoint of PDF format, as well as check the Slideshare upload.

Note that the Powerpoint format adds value in that sense that many of the slides come with additional explanations in the notes section.

If you are interested in more details I recommend visiting this post which (more...)

DOAG 2015 – "Oracle 12c Parallel Execution New Features" presentation material

Thanks to the many attendees that came to my presentation "Oracle 12c Parallel Execution New Features" at the DOAG conference 2015. You can download the presentation material here in Powerpoint of PDF format, as well as check the Slideshare upload.

Note that the Powerpoint format adds value in that sense that many of the slides come with additional explanations in the notes section.

If you are interested in more details I recommend visiting this post (more...)

12c Parallel Execution New Features: Parallel FILTER Subquery Evaluation – Part 2: Distribution Methods

Picking up from the first part of this instalment I'll focus in this post on the available distribution methods for the new parallel FILTER subquery feature.In this post I won't go into the details how the optimizer selects the distribution method automatically - this will be covered in the last part.Here I merely describe the different available methods and how to control them using the new PQ_FILTER hint, which is also mentioned in (more...)