Wrong Results

_rowsets_enabled – Apply patch and use the default

I while back I blogged about issues with “rowsets“, a new Oracle 12c feature which unfortunately had two known wrong result (WQR) bugs:

What does “rowsets” actually mean?

I’d like to thank Sankar, our Development manager for providing this explanation which sheds some light on how important this feature actually is:

“Rowsets is a SQL execution performance enhancement introduced in Oracle RDBMS release 12.1 and further extended in a future release of the Oracle Database.

Prior to 12.1, data processing in the SQL layer were

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What happened to the blog post about “12c parameters”?

Best Practice Hint

Two weeks ago I published a blog post about Parameter Recommendations for Oracle Database 12.1.0.2. And I took it down a day later. Why that?

I’ve got a lot of input from external sources for the “Parameter” blog post. And I’d like to thank everybody who contributed to it, especially Oracle ACE Ludovico Caldara.

Generally there was a bit of a misunderstanding internally about whether we should advertise” underscore parameters to cure some misbehavior of the database. In 99% of all cases I’d agree that underscores are not a good solution – especially when it comes …

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UPDATE: _rowsets_enabled in Oracle Database 12c

Please find a recent update here:


Last week I did post this entry with a strong recommendation to disable _rowsets_enabled in Oracle Database 12.1.0.2:

Today I can give you an update, more insight information and better workarounds.

Credits go to our DWH and Optimizer people (thanks to Hermann, Angela, Nigel and Mohammed).

When is the problem happening?

When a hash join operation receives rowsets from its right input but then produces one row at a time as output. This explains …

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Switch off “_rowsets_enabled” in Oracle Database 12c

Please find a recent update here:

and more important:


Twitter is a good thing. I get alerted on things I haven’t seen before. And sometimes some things are more than interesting.

This one is actually proven by Jonathan Lewis – and you can read all the details in Jonathan’s blog post here:

There seems to be a realistic chance to get wrong query results displayed (regardless of using SQL*Plus or a JDBC or any other client – see the comment by Stefan Koehler below …

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Things to consider BEFORE upgrading to Oracle 12.1.0.2 to AVOID poor performance and wrong results

Finally it got published on MyOracle Support (MOS) portal – one of the most helpful notes when we prep for an upgrade or a migration – and potentially the one with the longest title in MOS:

MOS Note: 2034610.1
Things to consider BEFORE upgrading to Oracle Database 12.1.0.2 to AVOID poor performance and wrong results 

Avoid Poor Performance and Wrong Results when upgrading to 12.1.0.2

One important note regarding these “Things to Consider” notes:
They will only list issues where either a fix or a proven workaround is available. They don’t list all open issues nor do they recommend every given underscore which may cure one or the other misbehavior.…

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Avoid Poor Performance and Wrong Results in 11.2.0.4

One of the best notes in MOS got refreshed last night for Oracle Database 11.2.0.4. It has not only information about performance bugs but also about the worst and meanest category of issues, wrong query result bugs. Those are the ones I call the “sleeping beauties” as you as a DBA usually won’t recognize them. It is recognized at the application level when the result of a report is incorrect or instead of 4 rows just 3 rows get delivered in the output. Therefore you should be highly aware of those and add this note to your

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