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Recursive 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...
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Ignoring 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...
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Delete Costs
One of the quirky little anomalies of the optimizer is that it’s not allowed to select rows from a table after doing an index fast full scan (index_ffs) even if it is obviously the most efficient (or,...
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SQL Plan Baselines
Here’s a thread from Oracle-L that reminded of an important reason why you still have to hint SQL sometimes (rather than following the mantra “if you can hint it, baseline it”). I have a query that...
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First Rows
Following on from the short note I published about the first_rows optimizer mode yesterday here’s a note that I wrote on the topic more than 2 years ago but somehow forgot to publish. I can get quite...
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Baselines
I’m not very keen on bending the rules on production systems, I’d prefer to do things that look as if they could have happened in a completely legal fashion, but sometimes it’s necessary to abuse the...
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push_pred – evolution
Here’s a query (with a few hints to control how I want Oracle to run it) that demonstrates the difficulty of trying to solve problems by hinting (and the need to make sure you know where all your...
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Filter Hash
One of the most irritating features of solving problems for clients is that the models I build to confirm my diagnosis and test my solutions often highlight further anomalies, or make me ask questions...
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Five Hints
This is the content of a “whitepaper” I wrote for my presentation “Five Hints for Optimising SQL” at the recent DOAG conference. Introduction Adding hints to production code is a practice to be avoided...
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Hinting
This is just a little example of thinking about hinting for short-term hacking requirements. It’s the answer to a question that came up on the Oracle-L listserver a couple of months ago (Oct 2015) and...
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Connect By
I received an email a couple of days ago that was a little different from usual – although the obvious answer was “it’s the data”. A connect by query with any one of several hundred input values ran in...
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Wrong Results
Just in – a post on the Oracle-L mailing lists asks: “Is it a bug if a query returns one answer if you hint a full tablescan and another if you hint an indexed access path?” And my answer is, I think:...
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Hinting
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...
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use_nl hint
In response to a recent lamentation from Richard Foote about the degree of ignorance regarding the clustering_factor of indexes I commented on the similar level of understanding of a specific hint...
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Parallel First_rows()
A recent posting on OTN raised the question of whether or not the “parallel” hint and the “first_rows(n)” hint were mutually incompatible. This reminded me that from time to time other posters on OTN...
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Parallelism
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...
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dbms_sqldiag
If you’re familiar with SQL Profiles and SQL Baselines you may also know about SQL Patches – a feature that allows you to construct hints that you can attach to SQL statements at run-time without...
View Articleopt_estimate
The opt_estimate hint is one of many that shouldn’t be used in end-user code and isn’t officially documented. Nevertheless – like so many other hints – it’s a hint that is hard to ignore when you see...
View ArticleConditional SQL – 5
Here’s a note that has been sitting around for more than 3 years (the draft date is Jan 2015), waiting for me to finish it off; and in that time we’ve got a new version of Oracle that changes the...
View ArticleHacking Profiles
Saturday’s posting about setting cursor_sharing to force reminded me about one of the critical limitations of SQL Profiles (which is one of those little reason why you shouldn’t be hacking SQL Profiles...
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