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June 2010

Beginner/Expert

I mentioned some time ago the presentation I did at Open World 2009 and Miracle Open World called “The Beginner’s Guide to becoming an Expert”. Over the last few months I’ve had a few people email me asking me if I’m going to post the presentation on my blog. The answer is no – because [...]

Optimizer Mode Mismatch Does Not Prevent Sharing of Child Cursor!?!?

The aim of this post is to describe a strange (buggy) situation that I observed recently. But before doing that, I shortly summarize what a parent cursor and a child cursor are as well as when they can be shared. By the way, I borrowed this description from the pages 20/21 of my book. Hence, [...]

Testing

The old chestnut of “optimal block size” came up on OTN again a few weeks ago, with someone asking for advice on how to do some testing to decide on the optimal block size for a database. The correct answer to this question is you don’t: you assume you are going to use the default [...]

Log File Sync and AWR – Not Good Bedfellows

I’m sure many of you are already enlightened enough about the dangers of ratios and averages, the most famous blunder of all being the ‘cache hit ratio’. It’s not that all ratios and averages are useless, but it is surely the case that most are useless, and that all can be misleading. This posting is about an example of a misleading average, one that hides the truth from the reader unless they look elsewhere.

I am actually a big fan of AWR (and Statspack). They are chock full of misleading averages and ratios, but that does not mean they don’t have value. Often it is more a case of reading what is absent from the report, reading between the lines and so forth, that makes them so valuable. I’m also frequently asked to fix problems that are no longer occurring – without the history that AWR provides this would be an impossible task, assuming that Active Session History would therefore also be unavailable.

Content thief

Do you happen to know Guenadi N Jilevski? If yes, then please tell him that stealing content is a bad idea. See: Select for update – unintended consequences – this is Mark Bobak’s Unintended Consequences Timing improvements in Oracle 11GR2 trace – this one is Alex Fatkulin’s Timing improvements in Oracle 11GR2 trace Oracle 11gR2 [...]

I wish .. (1)

I pointed out some time ago a few of the things in SQL Server that I would like to see in Oracle. Here’s a couple more: Executions Plans Insert, update, and delete execution plans show the indexed access paths used to check the side effects of referential integrity constraints. (I haven’t  checked to see what [...]

Bitmap Index Degradation Since 10g (Fix You)

As discussed in my earlier post on Bitmap Index Degradation After DML Prior To 10g, Oracle wasn’t particularly efficient in the manner it maintained Bitmap Indexes after DML operations. During insert operations, if an existing Bitmap index entry didn’t cover the rowid range of a new row to be inserted, Oracle would create a new Bitmap index entry with a [...]

Listagg function in 11g release 2

A new built-in function for string aggregation. June 2010