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Friday Philosophy – Sometime The Solution Has To Not Only Match The Problem But Also…

…The People!

When you design a system for end users, a good designer/developer considers the “UX” – User eXperience. The system has to be acceptable to the end user. This is often expressed as “easy to use” or “fun” or “Quick”. But in reality, the system can fail in all sort of ways but still be a success if the end user gets something out of using it. I’ve said it before and I’ll say it again and again until I give up on this career. In my opinion:

User Acceptance is the number one aim of any I.T. system.

Postgresql block internals

This blogpost is the result of me looking into how postgres works, and specifically the database blocks. The inspiration and essence of this blogpost comes from two blogs from Jeremiah Peschka: https://facility9.com/2011/03/postgresql-row-storage-fundamentals/ and https://facility9.com/2011/04/postgresql-update-internals/
I am using Oracle Linux 7u3 and postgres 9.6 (current versions when this blogpost was written).

Postgres is already installed, and a database cluster is already running. Let’s create a database ‘test’ for the sake of our tests:

$ createdb test

Once the database is created, logging on is done with ‘psql’:

It’s just bad code or bad design … most of the time

Some years ago I wrote an article for the UKOUG magazine called “Want a faster database – Take a drive on the M25”.  For those not familiar with the United Kingdom, the M25 is one of its busiest roads (M = “motorway”) and because it moves so much traffic, and runs so close to capacity, it has often been referred to as “the world’s largest car park”.  Many people have probably spent a good part of their lives on the M25 Smile  I used the M25 as a metaphor for how database professionals can focus on the wrong things when trying to solve a performance problem, such as:

“I’m stuck in traffic…perhaps a faster car will help”

ie, throwing CPU at a problem that is not CPU bound will not help things, or

“I’m stuck in traffic…it must be the width of the paint on the lane markings”

Unpivot

An interesting observation appeared recently as a side-channel on a question on the OTN database forum – how does Oracle execute an unpivot() operation. Here’s an example of such a query:

Instrumentation … not just for debugging

Yeah I know.  You’re probably thinking “Here’s another blog post from someone telling us how important it is to instrument our code, so we can get better debugging, better performance, blah blah blah”.

Quiz Night

If this is the closing section of thetkprof output from the trace file of a single end-user session that has a performance problem, what’s the most obvious deduction you can make about the cause of the problem, and what sort of action would you take next ?

SUM is better than DISTINCT

There is a good chance that (based on this blog post title) that you’re expecting a post on SQL, and that’s understandable. But I’ll come clean nice and early – that was just to lure you in Smile

The post is about SUM and DISTINCT, but not in the technical sense.

Guesswork

A recent posting on the OTN database forum described a problem with an insert (as select) statement that sometimes ran extremely slowly: nothing interesting yet, there could be plenty of boring reasons for that to happen. The same SQL statement (by SQL_ID) might take 6 hours to insert 300K rows one night while taking just a few minutes to insert 900K another night (still nothing terribly interesting).

New Events for Data Guard and Synchronous Redo Transport in 12c (2)

After the baseline has been established in the first part of this series it’s time to measure the effect of the network in this part. The second test will introduce an interesting feature: Using Linux’s own Traffic Shaper/Quality of Services module I will add a delay of 100ms to the Data Guard network interface card (NIC) to slow things down a little.

WARNING: this is of course a lab or VM-only situation. I can’t exert control over wire quality in my (own) switches, hence some software magic is needed on my virtual ones. This post is intended to be merely for educational purposes, not for use at work.

I am continuing to use the 2 node RAC 12.1.0.2.170117 primary database on Oracle Linux 7 with UEK 4 and an identical RAC to host my standby database.

New Events for Data Guard and Synchronous Redo Transport in 12c (1)

I may have said it before but I consider presenting and teaching a great way to expand one’s knowledge: first of all it requires me to really understand a subject. Secondly, when presenting, you get lots of interesting questions that can turn into blog posts like this one.

Lately I have been asked about the impact of synchronous log shipping to a physical standby database. I was sure there was an effect to be observed, depending most likely on the network latency between systems but I didn’t have any evidence I could pull out of the hat to back up my thoughts. So what better than trying! I also read that some of the events have changed in 12c, and wanted to make them visible. My environment is based on the 2 node RAC primary/2 node RAC standby configuration I wrote about in my previous posts.

Since their initial setup I upgraded the cluster to 12.1.0.2.170117 for Clusterware and RDBMS.