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May 2014

Sampling Performance Data on Oracle

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12c Hybrid Hash Distribution with Skew Detection / Handling - Failing

This is just an addendum to the previous post demonstrating one example (out of many possible) where the join skew handling feature fails. The test case setup is the same as in the previous post.As mentioned in the AllThingsOracle.com article and in the introduction of the previous post, the feature at present only applies to a rather limited number of scenarios. To wrap things up and to give an idea what can happen with that new feature, here's a three table join that actually makes use of the feature for one join, only to suffer from the skew problem in the next join that uses the same join expression, but doesn't qualify (yet) for the skew handling feature:

Combining Bloom Filter Offloading and Storage Indexes on Exadata

Here’s a little known feature of Exadata – you can use a Bloom filter computed from a join column of a table to skip disk I/Os against another table it is joined to. This not the same as the Bloom filtering of the datablock contents in Exadata storage cells, but rather avoiding reading in some storage regions from the disks completely.

So, you can use storage indexes to skip I/Os against your large fact table, based on a bloom filter calculated from a small dimension table!

Combining Bloom Filter Offloading and Storage Indexes on Exadata

Here’s a little known feature of Exadata – you can use a Bloom filter computed from a join column of a table to _skip_ disk I/Os against another table it is joined to. This not the same as the Bloom filtering of the datablock contents in Exadata storage cells, but rather avoiding reading in some storage regions from the disks completely.

So, you can use storage indexes to skip I/Os against your large fact table, based on a bloom filter calculated from a small dimension table!

Combining Bloom Filter Offloading and Storage Indexes on Exadata

Here’s a little known feature of Exadata – you can use a Bloom filter computed from a join column of a table to _skip_ disk I/Os against another table it is joined to. This not the same as the Bloom filtering of the datablock contents in Exadata storage cells, but rather avoiding reading in some storage regions from the disks completely.

So, you can use storage indexes to skip I/Os against your large fact table, based on a bloom filter calculated from a small dimension table!

How Exadata smartscans work

I guess everybody who is working with Oracle databases and has been involved with Oracle Exadata in any way knows about smartscans. It is the smartscan who makes the magic happen of full segment scans with sometimes enormously reduced scan times. The Oracle database does smartscans which something that is referred to as ‘offloading’. This is all general known information.

But how does that work? I assume more people are like me, and are anxious to understand how that exactly works. But the information on smartscans is extremely scarce. Of course there is the Oracle public material, which looks technical, but is little/nothing more than marketing. On My Oracle Support, I can’t find anything on the inner working. Even in the ‘Expert Oracle Exadata’ book (which I still regard as the best source of Exadata related information) there is no material on the mechanics of smartscans.

VirtualBox 4.3.12

VirtualBox 4.3.12 was been born yesterday. It’s a maintenance release, so a number of things have been fixed. The downloads and changelog are in the usual places.

Seems to install and work fine on Mac, Linux and Windows. :)

Cheers

Tim…

 

I want to be a simplifier – thanks Steve Jobs

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Any intelligent fool can make things bigger and more complex. It takes a touch of genius – and a lot of courage – to move in the opposite direction.

Subquery with OR

Prompted by a pingback on this post, followed in very short order by a related question (with a most gratifying result) on Oracle-L, I decided to write up a note about another little optimizer enhancement that appeared in 12c. Here’s a query that differs slightly from the query in the original article:

Enkitec + Accenture = Even More Awesomeness!

Enkitec is the best consulting firm for hands on implementation, running and troubleshooting your Oracle based systems, especially the engineered systems like Exadata. We have a truly awesome group of people here; many are the best in their field (just look at the list!!!).

This is why I am here.

This is also why Accenture approached us some time ago – and you may already have seen today’s announcement that Enkitec got bought!