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January 2018

Brand new year ? Brand new to installing Oracle ?

If you’ve stumbled across this blog trying to get started with a local installation of Oracle database for some personal development, then firstly, welcome!

Secondly, it can be a little daunting to get up to speed, so here’s a little video that will walk though the process of download, installing and getting up and running with Oracle database on a Windows laptop/desktop. It’s a lot easier than you think.

Announcement: Oracle Indexing Internals Seminars Coming to New Zealand in March 2018 (Shipyards of New Zealand)

I’m very pleased to announce I’ve now finalised some dates in New Zealand for my popular and highly acclaimed “Oracle Indexing Internals and Best Practices” seminar. They are: Wellington 12-13 March 2018 (Auldhouse: Level 8 Lumley House, 11 Hunter Street): Tickets and Registration Link Auckland 15-16 March 2018 (Karstens: Level 4, 205 Queen Street): Tickets and Registration Link […]

Unplug an Encrypted PDB (ORA-46680: master keys of the container database must be exported)

In the Oracle Database Cloud DBaaS you provision a multitenant database where tablespaces are encrypted. This means that when you unplug/plug the pluggable databases, you also need to export /import the encryption keys. You cannot just copy the wallet because the wallet contains all CDB keys. Usually, you can be guided by the error messages, but this one needs a little explanation and an example.

Here I’ll unplug PDB6 from CDB1 and plug it into CDB2

Boston Cloud SIG Session: Docker Demystified

Thank you to all for coming to my session for North East Oracle User Group (Boston), more specifically the first ever Cloud SIG meetup a Burlington, MA on Jan 18th. It was heartening to see a 100+ crowd for a session that spanned all the way up to 9:30 PM on a week day. I sincerely hope you enjoyed it.

Here is the presentation for you to download:

Nested MVs

A recent client was seeing a very large redo penalty from refreshing materialized views. Unfortunately they had to be refreshed very frequently, and were being handled with a complete refresh in atomic mode – which means delete every row from every MV then re-insert every row.  The total redo was running at about 5GB per hour, which wasn’t a problem for throughput, but the space for handling backup and recovery was getting a bit extreme.

The requirement consisted of two MVs which extracted and aggregated row and column subsets in two different ways from a single table; then two MVs that aggregated one of the first MVs in two different ways; then two MVs which each joined one of the first level MVs to one of the scond level MVs.

CDB Views and Query Optimizer Cardinality Estimations

Today I faced a performance problem caused by a bad cardinality estimation involving a CDB view in a multitenant environment. While solving the problem I did a number of observations that I try to summarize in this blog post.

First of all, when checking the execution plan of a query already running for more than two hours, I noticed that, in the execution plan, neither the referenced CDB view nor one of its underlying objects were referenced. The following query (and its execution plan) executed while connect to the CDB illustrates (I also added the output to show you the difference it that area):

Column Stats

I’ve made several comments in the past about the need for being selective when gathering objects statistics with particular reference to the trade-offs when creating histograms. With Oracle 12c it’s now reasonably safe (as far as I’m concerned) to set a method_opt as a table preference that identifies columns where you expect to see Frequency or (pace the buggy behaviour described in a recent post) a Top-N histograms. The biggest problem I have is that I keep forgetting the exact syntax I need – so I’ve written this note more as a reminder to myself than anything else.

GDPR ‘Murica!

Just over a year ago, an alarm of emails, posts and projects arose in Europe surrounding the General Data Protection Regulation, also known with the acronym, GDPR.  It was as if someone had poked the sleeping bear of IT and woke it and boy, was it grumpy.

Histogram Hassle

I came across a simple performance problem recently that ended up highlighting a problem with the 12c hybrid histogram algorithm. It was a problem that I had mentioned in passing a few years ago, but only in the context of Top-N histograms and without paying attention to the consequences. In fact I should have noticed the same threat in a recent article by Maria Colgan that mentioned the problems introduced in 12c by the option “for all columns size repeat”.

So here’s the context (note – all numbers used in this example are approximations to make the arithmetic obvious).  The client had a query with a predicate like the follwing:

Spectre and Meltdown on Oracle Public Cloud UEK – LIO

In the last post I published the strange results I had when testing physical I/O with the latest Spectre and Meltdown patches. There is the logical I/O with SLOB cached reads.