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Result Cache

Yesterday I thought I’d spend half an hour before breakfast creating a little demonstration of a feature; some time about midnight I felt it was time to stop because I’d spent enough time chasing around a couple of bugs that produced wrong results in a variety of ways. Today’s short post is just little warning: be VERY careful what you do with the PL/SQL result cache – if you use the results of database queries in the cache you may end up with inconsistent results in your application. Here’s one very simple example of what can go wrong, starting with a little script:

The Fundamental Challenge of Computer System Performance

The fundamental challenge of computer system performance is for your system to have enough power to handle the work you ask it to do. It sounds really simple, but helping people meet this challenge has been the point of my whole career. It has kept me busy for 26 years, and there’s no end in sight.

Capacity and Workload

Our challenge is the relationship between a computer’s capacity and its workload. I think of capacity as an empty box representing a machine’s ability to do work over time. Workload is the work your computer does, in the form of programs that it runs for you, executed over time. Workload is the content that can fill the capacity box.

Friday Philosophy – On “Being the Expert”

Working as a recognised expert at something is a little…strange, I find.

I had an assignment this week to go visit a client, have a look at a performance issue and find out the root cause. I was also to at least come up with suggested resolutions with the ideal aim of giving them a proven fix they could implement. All to be done in two to three days. This is pretty standard fayre when you are putting yourself forward as some sort of expert in something. And it is not always an easy thing to do – for more reasons than you might expect.

[Oracle] Insights into SQL hints - Embedded global and local hints and how to use them


The idea for this blog post started a few weeks ago when i had to troubleshoot some Oracle database / SQL performance issues at client site. The SQL itself included several views and so placing hints (for testing purpose) into the views was not possible, especially as the views were used widely and not only by the SQL with the performance issue. In consequence this blog post is about the difference between embedded global and local hints and how to use them.


Friday Philosophy – Being Rejected by the Prom Queen

If you follow me on twitter (and if you are on twitter, why would you *not* follow me :-) See Twitter tag on right of page -> ) you will know what the title is all about. I posted the below on my twitter feed a few weeks ago:

Submitting to speak at #OOW15 is like asking out prom queens. You live in hope – but expect rejection :-)

{BTW if prom queens are not your thing and you would rather be asking out the captain of the football/ice hockey/chess team, the vampire slayer or whatever, just substitute as you see fit.}


Which piece of code will be faster (clue – the table in question has no indexes):

Option 1 – pure SQL

update join1 set
        data = data||'#'
where   key_1=500
and     key_2=23851
and     key_3=57012
and     key_4=521
and     key_6=1
and     key_7=23352

Option 2 – a silly PL/SQL row by row approach:

Investigating the full table direct path / buffered decision.

A lot of blogposts and other internet publications have been written on the full segment scan behaviour of a serial process starting from Oracle version 11gR2. This behaviour is the Oracle engine making a decision between scanning the blocks of a segment into the Oracle buffercache or scanning these blocks into the process’ private process global area (PGA). This decision is even more important on the Exadata platform, because the Oracle engine must have made the decision to read the blocks into the process’ PGA in order to be able to do a smartscan. This means that if you are on Oracle 11gR2 already, and thinking about using the Exadata platform, the wait event ‘direct path read’ gives you an indication on how much potentially could be offloaded on Exadata, if you keep all the settings the same.

[Oracle] DB Optimizer Part XII - Revealing SQL Plan Directive details for existing/loaded cursor from CBO (and SQL Dynamic Sampling Services) trace


The idea for this blog post is based on a recent Twitter discussion with Martin Berger, Martin Bach and Mauro Pagano about revealing SQL Plan Directive details for an existing cursor as walking through the standard Oracle data dictionary views can be very time consuming/slow and there are still some details missing about the dynamic sampling task itself, even if you have found what you are looking for.


With Modern Storage the Oracle Buffer Cache is Not So Important.

With Oracle’s move towards engineered systems we all know that “more” is being done down at the storage layer and modern storage arrays have hundreds of spindles and massive caches. Does it really matter if data is kept in the Database Buffer Cache anymore?

Yes. Yes it does.

Time for a cool beer

Time for a cool beer

With much larger data sets and the still-real issue of less disk spindles per GB of data, the Oracle database buffer cache is not so important as it was. It is even more important.

If you use swingbench take note of sbutil

This is going to be a very short post for a change. I have used Swingbench extensively and really love the tool. Many thanks to Dominic Giles!

Recently he announced a new tool on his blog that you can use to inflate your data volume. So instead of using the “-scale” argument when executing oewizard you can just keep the defaults and later on create as much data as you like. Here is an example, the reason for this post.

Setting the Scene