Index Hash
I’m afraid this is one of my bad puns again – an example of the optimizer making a real hash of the index hash join. I’m going to create a table with several indexes (some of them rather similar to...
View ArticleRecursive subquery factoring
This is possibly my longest title to date – I try to keep them short enough to fit the right hand column of the blog without wrapping – but I couldn’t think of a good way to shorten it (Personally I...
View ArticleIgnoring Hints
Does Oracle ignore hints – not if you use them correctly, and sometimes it doesn’t ignore them even when you use them incorrectly!Here’s an example that I’ve run on 11.2.0.4 and 12.1.0.1 create table...
View ArticleHinting
A posting on the OTN database forum a few days ago demonstrated an important problem with hinting – especially (though it didn’t come up in the thread) in the face of upgrades. A simple query needed a...
View ArticleParallelism
Headline – if you don’t want to read the note – the /*+ parallel(N) */ hint doesn’t mean a query will use parallel execution, even if there are enough parallel execution server processes to make it...
View ArticleAppend hint
One of the questions that came up on the CBO Panel Session at the UKOUG Tech2018 conference was about the /*+ append */ hint – specifically how to make sure it was ignored when it came from a 3rd party...
View ArticleIgnoring Hints
One of the small changes (and, potentially big but temporary, threats) in 18.3 is the status of the “ignore hints” parameter. It ceases to be a hidden (underscore) parameter so you can now officially...
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 ArticleIndex Hints
I’ve lost count of the number of times I’ve reminded people that hinting (correctly) is hard. Even the humble /*+ index() */ hint and its close relatives are open to misunderstanding and accidental...
View ArticleIndex Hints
At the end of the previous post on index hints I mentioned that I had been prompted to complete a draft from a few years back because I’d been sent an email by Kaley Crum showing the optimizer ignoring...
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