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October 2018

“Hidden” Efficiencies of Non-Partitioned Indexes on Partitioned Tables Part I (The Jean Genie)

When it comes to indexing a partitioned table, many automatically opt for Local Indexes, as it’s often assumed they’re simply easier to manage and more efficient than a corresponding Global Index. Having smaller index structures that are aligned to how the table is partitioned certainly has various advantages. The focus in this little series is on […]

Ansible tips’n’tricks: a different output option

When running ansible scripts, occasionally you wonder why a given task has failed. I found out more than once that it’s commonly a problem with the script, not the engine ;) Finding out exactly where in the script I made the mistake can be more of a challenge.

With the default ansible settings, output can be a bit hard to read. Consider this example: I do quite a bit of patching in my lab, and this almost always requires an upgrade of OPatch (d’oh!). So instead of connecting to each of my hosts and performing the same unzip command over and over again, I thought of using something else. Why not use ansible for this task? It won’t get tired copying/unzipping OPatch to all the destinations I indicate in my configuration. And it won’t introduce a mistake when dealing with the fifth ORACLE_HOME on the third server…

Join Cardinality

Following up my “Hacking for Skew” article from a couple of days ago, Chinar Aliyev has written an article about a method for persuading the optimizer to calculate the correct cardinality estimate without using any undocumented, or otherwise dubious, mechanisms.

Easy as pi…. hole.

A slight digression from my normal database-focussed content today Smile

In internet bandwidth and latency strapped Perth (Western Australia), every last drop of internet counts. Recently I stumbled upon this blog post by Troy Hunt about using a Raspberry Pi to serve as a local DNS to filter out unnecessary content. Personally, I don’t care about the content as such (I don’t like ads, but I do acknowledge that they are generally a necessary “evil”), but for me it is about getting the most performance out of my lowly internet connection until the technology evolves in Australia.

Repairing a Linux host with disk/filesystem issues

This is a writeup on some scenario’s for disk issues that you could encounter when running linux systems. The linux systems I am talking about are centos/redhat/oracle (EL) version 7 systems. This is just a writeup for myself to know how to deal with different scenario’s, hopefully other people find this interesting too. I don’t believe there is any difference for the below scenario’s and resolutions between running physical/bare metal, virtualised or in the cloud, provided you can get access the you need (like the BIOS POST console). The test configuration uses (legacy) MBR (meaning non UEFI), grub2 as boot loader, LVM for meta devices (you don’t want to run without LVM!!) and XFS for the filesystems.

Unindexed Foreign Keys in Oracle and PostgreSQL

A new blog post on the Databases at CERN blog: verifying (with pgSentinel) that PostgreSQL does not lock full tables like Oracle does when the foreign key is not indexed.

https://db-blog.web.cern.ch/blog/franck-pachot/2018-09-unindexed-foreign-keys-oracle-and-postgresql

Unindexed Foreign Keys in Oracle and PostgreSQL

SLOB Chewed Up All My File System Space and Spit It Out. But, Why?

This is a quick blog post in response to a recent interaction with a SLOB user. The user reached out to me to lament that all her file system space was consumed as the result of a SLOB execution (runit.sh). I reminded her that runit.sh will alert to possible derelict mpstat/iostat/vmstat processes from an aborted SLOB test. If these processes exist they will be spooling their output to unlinked files.

The following screen shot shows what to expect if a SLOB test detects potential “deadwood” processes. If you see this sort of output from runit.sh, it’s best to investigate whether in fact they remain from an aborted test or whether there are other users on the system that left these processes behind.

 

Ansible tips’n’tricks: a new series

In the past few months I have spent considerable amounts of time working on new technology (new as in “new to me”) and one of the things I have found a great interest-and more importantly-use cases with, is ansible. I don’t know how it is with you, but if I don’t have a practical use case for using a technology I find it hard to get familiar with it.

Ansible is a fantastic piece of technology with decent documentation and great community support. I didn’t find writing ansible code too hard. Most of the problems I have encountered while learning how to write ansible playbooks have-in some way or another-already been encountered and solved by other users. It does require a bit of Internet research, and you will need check if the solution offered somewhere on the Internet is still valid with the current version of the tool. As with a lot of other software, ansible evolves at a rather quick pace and keeping up can be a bit of a challenge.

Using the Query Cache for good performance in #Exasol

The result of a query can be cached in Exasol to the effect that repeated identical queries complete in no time. This feature has been introduced in version 5 and is enabled by default.

SQL_EXA> select session_value,system_value  from exa_parameters where parameter_name='QUERY_CACHE';
EXA: select session_value,system_value  from exa_parameters where parameter...

SESSION_VALUE        SYSTEM_VALUE
-------------------- --------------------
ON                   ON

1 row in resultset.

The Query Cache can be (de-)activated on the session level as well as on the system level.