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32K Columns
Oracle 12c has increased the maximum length of character-based columns to 32K bytes – don’t get too excited, they’re stored out of lines (so similar in cost to LOBs) and need some modification to the...
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Rowids
I have, in the past, used the dbms_rowid package to create rowids from block addresses (typically faking the first and last rowids that could appear in an extent); but I’ve just been sent a piece of...
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Buffer Pins
Sometimes you get some questions on OTN lead to very geeky investigations. Here’s one that came up a while ago that started with a reasonable observation about recursive subquery factoring, then...
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Bitmap Question
This question came up on the OTN database forum a couple of months ago: “Why doesn’t Oracle allow you to create globally partitioned bitmap indexes?” The obvius answer is “It just doesn’t, okay.” But...
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dbms_space usage
Just throwing out a brief comment (one of my many draft notes that I don’t have time to complete) about the dbms_space package. You’re probably familiar with this package and how, for ASSM segments, it...
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LOB changes
It’s always useful to collect baseline information – especially when it helps you notice that the baseline has moved in a way that might explain the next performance problem you see. Here’s an example...
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CR Trivia
Everybody “knows” that when you do a tablescan of a table that it starts with two buffer gets on the segment header, though older versions (ca. 8i and earlier) of Oracle used to do 4 buffer gets on the...
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xDollar
I see Tanel has just started a short series of articles on where the X$ data come from so, for those with time on their hands (?anyone?) here’s a little script I wrote a few years ago when I wanted to...
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Single block reads
When a “cache read” tablescan (or index fast full scan) takes place we generally expect to see waits on “db file scattered read” as Oracle performs multi-block reads to do the scan. But we all know...
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Clustaghhh!
It doesn’t matter which bit of Oracle technology you want to use, eventually someone, somewhere, runs into the special case where something nasty happens. Here’s an edge case for people using (index)...
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IOT Catalogue
I’ve just added a catalogue of Richard Foote’s articles on IOTs to the list I made a couple of years ago of Martin Widlake’s articles, so this is just a temporary note to point people to the updated...
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Row Migration
At one of the presentations I attended at RMOUG this year the presenter claimed that if a row kept increasing in size and had to migrate from block to block as a consequence then each migration of that...
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Index Compression – aargh
The problem with telling people that some feature of Oracle is a “good thing” is that some of those people will go ahead and use it; and if enough people use it some of them will discover a hitherto...
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Indexing LOBs
Many years ago, possibly when most sites were still using Oracle 8i, a possible solution to a particular customer problem was to create a function-based index on a CLOB column using the...
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Auto Sample Size
In the past I have enthused mightily about the benefits of the approximate NDV mechanism and the benefit of using auto_sample_size to collect statistics in 11g; however, as so often happens with Oracle...
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Duplicate database
One of the people attending my seminar in Munich last week has emailed me some details about a nasty little surprise you may get if you’re thinking about TSPITR (tablespace point in time recovery), and...
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12c Temporary
Just one of those little snippets to cover something new and remind of something old. A single session can now have three (or more) temporary tablespaces in use at the same time for different reasons....
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Temporary Segments
A question came up on oracle-l recently about identifying which temporary segment in v$tempseg_usage is associated with which global temporary table. Here’s my answer: v$tempseg_usage is a synonym for...
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RLS bug
RLS – row level security, aka VPD (virtual private database) or FGAC (fine grained access control) has a critical bug in 11g. The bug is unpublished, but gets mentioned in various other documents, so...
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RLS bug
RLS – row level security, aka VPD (virtual private database) or FGAC (fine grained access control) has a critical bug in 11g. The bug is unpublished, but gets mentioned in various other documents, so...
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