Index Bouncy Scan 3
This is a follow-up to a problem I had with yesterday’s example of using recursive CTEs to “bounce” along a multi-column index to pick out the unique set of combinations of the first two columns. Part...
View ArticleDescending bug
Following on from Monday’s posting about reading execution plans and related information, I noticed a question on the ODC database forum asking about the difference between “in ({list of values})” and...
View ArticleColumn Groups
Sometimes a good thing becomes at bad thing when you hit some sort of special case – today’s post is an example of this that came up on the Oracle-L listserver a couple of years ago with a question...
View Articlenum_index_keys
The title is the name of an Oracle hint that came into existence in Oracle 10.2.0.3 and made an appearance recently in a question on the rarely used“My Oracle Support” Community forum (you’ll need a...
View ArticleIndex rebuild bug
I tweeted a reference yesterday to a 9 year old article about index rebuilds, and this led me on to look for an item that I thought I’d written on a related topic. I hadn’t written it (so there’s...
View ArticleDescribe Upgrade
Here’s an odd little change between Oracle versions that could have a stunning impact on the application performance if the thing that generates your client code happens to use an unlucky selection of...
View ArticleDanger – Hints
It shouldn’t be possible to get the wrong results by using a hint – but hints are dangerous and the threat may be there if you don’t know exactly what a hint is supposed to do (and don’t check very...
View ArticleANSI bug
The following note is about a script that I found on my laptop while I was searching for some details about a bug that appears when you write SQL using the ANSI style format rather than traditional...
View ArticleGlitches
Here’s a question just in from Oracle-L that demonstrates the pain of assuming things work consistently when sometimes Oracle development hasn’t quite finished a bug fix or enhancement. Here’s the...
View ArticleMulti-table
Here’s a problem (and I think it should be called a bug) that I first came across about 6 years ago, then forgot for a few years until it reappeared some time last year and then again a few days ago....
View ArticleIOT Bug
Here’s a worrying bug that showed up a couple of days ago on the Oracle-L mailing list. It’s a problem that I’ve tested against 12.2.0.1 and 19.3.0.0 – it may be present on earlier versions of Oracle....
View ArticleFlashback Archive
A classic example of Oracle’s “mix and match” problem showed up on the Oracle Developer Forum a few days ago. Sometimes you see two features that are going to be really helpful in your application –...
View ArticleDrop Column bug
When I was a child I could get lost for hours in an encyclopedia because I’d be looking for one topic, and something in it would make me want to read another, and another, and …The same thing happens...
View ArticleGroup by Elimination
Here’s a bug that was highlighted a couple of days ago on the Oracle Developer Community forum; it may be particularly worth thinking about if if you haven’t yet got up to Oracle 12c as it appeared in...
View ArticleANSI flashback
I am seeing “traditional” Oracle SQL syntax being replaced by “ANSI”-style far more frequently than I used to – so I thought I’d just flag up another reminder that you shouldn’t be too surprised if you...
View ArticleChar problems
The semantics of comparing character columns of different types can lead to some confusion, so before I get into the main body of this note here’s a little test based on a table with one row: create...
View ArticleJoin Elimination bug
It is possible to take subquery factoring (common table expressions / CTEs) too far. The most important purpose of factoring is to make a complex query easier to understand – especially if you can...
View Articlemaxquerylen
The view v$undostat is a view holding summary information about undo activity that can be used by the automatic undo mechanism to deal with optimising the undo retention time (hence undo space...
View ArticleFake Baselines – 2
Many years ago (2011) I wrote a note describing how you could attach the Outline Information from one query to the SQL_ID of another query using the official Oracle mechanism of calling...
View ArticleSequence Costs
You’re probably aware of the “identity” construct that appeared in 12.1 and uses Oracle’s sequence mechanism to model the Autonumber or Identity columns that other databases have. A posting from Clay...
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