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July 2018

Data Guard: always set db_create_file_dest on the standby

By Franck Pachot

The file name convert parameters are not dynamic and require a restart of the instance. An enhancement request was filled in 2011. I mentioned recently on Twitter that it can be annoying with Active Data Guard when a file on the primary server is created on a path that has no file name conversion. However, Ian Baugaard mentioned that there is a workaround for this specific case because db_create_file_dest is dynamic:

Historic Stats

If you want to examine historic object stats Oracle gives you a few procedures in the dbms_stats package to compare sets of stats captured at two different time periods, but there’s no view that you can query to get an idea of how a table’s stats have changed over time. This is a problem that can be addressed when you discover two things:

  • There are views to report pending table, index, column and histogram stats.
  • Pending stats are stored stored as “historic” stats with a future date.

Once you’ve spotted the second detail, you can acquire the SQL to generate the pending stats views:

Stays One Step Ahead With Certification

I started with Oracle certifications at the time of 8i. I had good experience on development side, but more limited on operation side. The certification, 4 exams at that time, was the way for me to prove my knowledge on *all* database domains, including operations. After that, I was able to find position on operation DBA side more easily without the ‘did you administrate big critical databases in production?’. It was clearly more liberty in job seeking.

PostgreSQL Active Session History (ash): welcome to the pg_active_session_history view (part of the pgsentinel extension)

Why active session history?

What if you could record and query an history of the active sessions? Would not it be useful for performance tuning activities?

With active session history in place you could have a look to the “near” past database activity. You could answer questions like:

  • What wait events type were taking most time?
  • What wait events were taking most time?
  • Which application name was taking most time?
  • What was a session doing?
  • What does a SQL statement wait for?
  • How many sessions were running in CPU?
  • Which database was taking most time?
  • Which backend type was taking most time?
  • On which wait event was the session waiting for?
  • And so on….

How does it look like?

Let’s have a look to the pg_active_session_history view (more details on how to create it later on):

My tmux scripts to script demos

By Franck Pachot

When I did my first demo using tmux, I have written the following blog post to explain it: Since then I’ve done all my demos with this technique. My scripts and configuration files have evolved now I have uploaded the on GitHub to share them:

Oracle archivelog deletion policy

Here are my posts on the dbi-services blog about archivelog deletion policy.

The deletion policy on a dataguard configuration should be:

for the site where you don’t backup. It can be the standby or the primary.


for the site where you do the backups. It can be the primary or the standby.

Some related posts:

Data Modeling, Dates and DAX

Presenting data in the format to ease visualization is required for any BI product.  Power BI provides much of this with Data Analysis Expressions, (DAX).   As a DBA, I admit to cringing every time a reference was made how similar it is to functions in Excel or other non-database platforms.  I’m a DBA and I naturally am going to see data at a much larger, more complex level.  I love the simplicity of DAX, which granted me the ability to acquire basic skills using it in just a day, but considering Power BI’s ability to pull from multiple data sources, including SQL Server, Oracle, PostgreSQL and even JSON files, the comparison to Excel left me feeling, well, ‘meh.’  </p />

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18c: Order by in WITH clause is not preserved

By Franck Pachot

For a previous post I’ve run on 18c a script of mine to get the V$MYSTAT delta values between two queries. This script (new version available on GitHub) generates the queries to store some values and subtract them on the next execution. But I had to fix it for 18c because I relied on some order by in a CTE which is lost in 18c.
The idea was to get the statistic names in a Common Table Expression (CTE):

with stats as (
select rownum n,stat_id,name from (select stat_id,name from v$statname where name in (&names) order by stat_id)

and query it from different parts of the UNION ALL which generates the script:

select 'select ' from dual
union all

#Exasol Fail-Safety explained

The building blocks of an Exasol cluster are commodity Intel servers like e.g. Dell PowerEdge R740 with 96 GB RAM,12 x 1.2 TB SAS Hot-plug hard-drives and 2 x 10Gb Ethernet Cards for the private network. That’s sufficient to deliver outstanding performance combined with high availability. The picture below shows a 4+1 cluster, one of our most popular configurations:

Complex materialized views and fast refresh

Just a quick discovery that came across the AskTOM “desk” recently. We have an outstanding bug in some instances of fast refresh materialized views when the definition of the materialized view references a standard view.

Here’s a simple demo of the issue – I’ll use a simplified version of the EMP and DEPT tables, linked by a foreign key in the usual way: