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October 2018

ODC Appreciation Day–LOB compression

LOBs tend to be large. Well duh…it’s right there in the name! “Large Object”. So one of the cool things I like with the SECUREFILE option in recent releases of Oracle Database is the ability to compress LOBs. Here’s a quick demo of that in action:

Hybrid Fake

Oracle 12c introduced the “Hybrid” histogram – a nice addition to the available options and one that (ignoring the bug for which a patch has been created) supplies the optimizer with better information about the data than the equivalent height-balanced histogram. There is still a problem, though, in the trade-off between accuracy and speed: just as it does with height-balanced histograms when using auto_sample_size Oracle samples (typically) about 5,500 rows to create a hybrid histogram, and the SQL it uses to generate the necessary summary is essentially an aggregation of the sample, so either you have a small sample with the risk of lower accuracy or a large sample with an increase in workload.

Eye, Eye, Cap’n

By the time you read this I will have had the lenses in both my eyes replaced, so I won’t be staring at a computer screen for a while – and that means, in particular, I won’t be doing any further investigation into join cardinality for a while.

For those who might otherwise feel deprived of my exquisite prose I have, however, prepared a couple of lightweight articles for automatic launching over the next few days. But please don’t expect any prompt follow-ups if you add comments or send email over the next couple of weeks.

OpenWorld Tuesday … yep, still screwed

Well…I’m only planning day 2 of OpenWorld and already I’ve pretty much given up on being able to see all the talks that I want to see Smile The challenge for me will be trying to coax those people that are inside Oracle to give me a synopsis of their talk after the event. That is probably my best hope here.

In any event, if you’re a database person like me, here’s my tips for what I’ll be trying to sneak into on Tuesday.

Tuesday morning

An Insider’s Guide to Oracle Autonomous OLTP Database Cloud [TRN3979]
Maria Colgan, master product manager at Oracle Corporation , Oracle
Robert Greene, Senior Director, Product Management, Oracle
Tuesday, Oct 23, 11:15 a.m. – 12:00 p.m. | Moscone West – Room 3003

which overlaps with

No /proc/diskstats Does Not Track **Your** Physical I/O Requests

You have applications that scan disk using large sequential reads so you take a peek at /proc/diskstats (field #4 on modern Linux distributions) before and after your test in order to tally up the number of reads your application performed. That’s ok. That’s also a good way to get erroneous data.

Your application makes calls for I/O transfers of a particular size. The device drivers for your storage might not be able to accommodate your transfer request in a single DMA and will therefore “chop it up” into multiple transfers. This is quite common with Fibre Channel device drivers where, for example, I/O requests larger than, say, 256KB get rendered into multiple 256KB transfers in the kernel.

This is not a new phenomenon. However, folks may not naturally expect how stats in /proc/diskstats reflect this phenomenon.

Join Cardinality – 3

In the previous posting I listed the order of precision of histograms as:

“Hidden” Efficiencies of Non-Partitioned Indexes on Partitioned Tables Part II (Aladdin Sane)

In Part I of this series, I highlighted how a Non-Partitioned Global Index on a Partitioned Table is able to effectively perform “Partition Pruning” by reading only the associated index entries to access just the table blocks of interest from relevant table partitions when the table partitioned keys are specified in an SQL Predicate. Understanding […]

Random Upgrade

Here’s a problem that (probably) won’t affect the day to day running of most systems – but it could be a pain in the backside for people who write programs to generate repeatable test data. I’m not going to say much about the problem, just leave you with a test script.

Use Azure CLI…I Beg You…

#333333; cursor: text; font-family: -apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Oxygen-Sans,Ubuntu,Cantarell,'Helvetica Neue',sans-serif; font-size: 16px; font-style: normal; font-variant: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-decoration: none; text-indent: 0px; text-transform: none; -webkit-text-stroke-width: 0px; white-space: normal; word-spacing: 0px;">Azure CLI made me feel right at home after working at Oracle in the Enterprise Manager CLI, (EMCLI)  The syntax is simple, powerful and allows an interface to manage Azure infrastructure from the command line, scripting out complex processing that would involve a lot of time in the user interface.
 

	

Join Cardinality – 2

In the previous note I posted about Join Cardinality I described a method for calculating the figure that the optimizer would give for the special case where you had a query that: