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Testing Oracle SQL online

Want to test some DDL, a query, check an execution plan? You need only a browser. And you can copy-paste, or simply link, your test-case in a forum, a tweet, an e-mail, a tweet. Here is a small list (expecting to grow from your comments) of free online services which can run with an Oracle Database: SQL Fiddle, Rextester, db<>fiddle and Oracle Live SQL

SQL Fiddle

SQL Fiddle let you build a schema and run DDL on the following databases:

  • Oracle 11gR2
  • Microsoft SQL Server 2014
  • MySQL 5.6
  • Postgres 9.6 and 9.3
  • SQLLite (WebSQL and SQL.js)

As an Oracle user, the Oracle 11gR2 is not very useful as it is a version from 2010. But there’s a simple reason for that: that’s the latest free version – the Oracle XE Edition. And a free online service can run only free software. Now that Oracle plans to release an XE version every year, this should be better soon.

Video: Oracle X$TRACE, Wait Event Internals and Background Process Communication

I have uploaded the the video of my Secret Hacking Session: Oracle X$TRACE, Wait Event Internals and Background Process Communication to my Oracle performance & troubleshooting Youtube channel.

The slides are in Slideshare.



Explain Plan format

The DBMS_XPLAN format accepts a lot of options, which are not all documented. Here is a small recap of available information.

The minimum that is displayed is the Plan Line Id, the Operation, and the Object Name. You can add columns and/or sections with options, such as ‘rows’, optionally starting with a ‘+’ like ‘+rows’. Some options group several additional information, such ‘typical’, which is also the default, or ‘basic’, ‘all’, ‘advanced’. You can choose one of them and remove some columns, with ‘-‘, such as ‘typical -rows -bytes -cost -plan_hash -predicate -remote -parallel -partition -note’. Finally, from an cursor executed with plan statistics, you can show all execution statistics with ‘allstats’, and the last execution statistics with ‘allstats last’. Subsets of ‘allstats’ are ‘rowstats’, ‘memstats’, ‘iostats’, buffstats’.

Of course, the column/section is displayed only if the information is present.


I had a sudden urge to go a bit meta – so here’s a relative frequency histogram of my observations of the general use of histograms in Oracle:


Linux 101 for SQL Server DBAs, Part II

Linux has become a clear area of required learning for SQL Server DBAs in as of SQL Server 2016.  What?  You didn’t read that right, you say?  Yes, while a Linux edition was introduced in SQL Server 2016, it was pushed to the forefront with the release of 2017, along with Python in a push towards stronger DevOps initiatives, (one OS to rule them all!) and Python to assist with even stronger analytics trends.

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

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.

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: