Histogram Limit
A surprising question came up on OTN a couple of days ago:Why does a query for “column = 999999999999999999” run slower than a query for “column > 999999999999999998” (that’s 18 digit numbers, if...
View ArticleWrong Results
Here’s a little gem in 12c that arrived in my email a few days ago: a query where the result depends on the SQL*Plus arraysize!The email had a short description, and a script to create a small data set...
View ArticleTable Expansion
I’ve often found that while I’m investigating one Oracle feature I get waylaid by noticing anomalies in other parts of the code. I was caught by one of these events a little while ago while...
View ArticleANSI bug
In almost all cases the SQL you write using the ANSI standard syntax is tranformed into a statement using Oracle’s original syntax before being optimised – and there are still odd cases where the...
View ArticleNLS Mess
The Oracle database has all sorts of little details built into it to help it deal with multi-national companies, but since they’re not commonly used you can find all sorts of odd “buggy” bits of...
View ArticleSubquery Effects
Towards the end of last year I used a query with a couple of “constant” subqueries as a focal point for a blog note on reading parallel execution plans. One of the comments on that note raised a...
View ArticlePartitioned Bitmap Join
If you don’t want to read the story, the summary for this article is:If you create bitmap join indexes on a partitioned table and you use partition exchanges to load data into the table then make sure...
View ArticleWrong 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:...
View ArticleCursor_Sharing problem
Here’s a possible bug (though maybe “not a bug”) that came up over the weekend on the OTN database forum. An application generating lots of “literal string” SQL was tested with cursor_sharing set to...
View ArticleInvisible Bug
At this Wednesday’sOracle Midlands event someone asked me if Oracle would use the statistics on invisible indexes for the index sanity check. I answered that there had been a bug in the very early days...
View ArticleDML and Bloom
One of the comments on my recent posting about “Why use pl/sql bulk strategies over simple SQL” pointed out that it’s not just distributed queries that can change plans dramatically when you change...
View ArticleLost Concatenation
This note models one feature of a problem that came up at a client site recently from a system running 12.1.0.2 – a possible bug in the way the optimizer handles a multi-column in-list that can lead to...
View ArticleParallel_index hint
Prompted by a recent OTN posting I’ve dug out from my library the following demonstration of an anomalty with the parallel_index() hint. This note is a warning about how little we understand hints and...
View ArticleJoin Elimination 12.2
From time to time someone comes up with the question about whether or not the order of tables in the from clause of a SQL statement should make a difference to execution plans and performance. Broadly...
View Articlemin/max Upgrade
A question came up on the OTN database forum a little while ago about a very simple query that was taking different execution paths on two databases with the same table and index definitions and...
View ArticleJoin Elimination Bug
A few years ago a bug relating to join elimination showed up in a comment to a post I’d done about the need to keep on testing and learining. The bug was visible in version 11.2.0.2 and, with a script...
View ArticleASSM argh!
Here’s a problem with ASSM that used to exist in older versions of Oracle had disappeared by 11.2.0.4 and then re-appeared in 12.1.0.2 – disappearing again by 12.2.0.1. It showed up on MoS a few days...
View ArticleNested MVs
A recent client was seeing a very large redo penalty from refreshing materialized views. Unfortunately they had to be refreshed very frequently, and were being handled with a complete refresh in atomic...
View ArticleInterval Partition Problem
Assume you’ve got a huge temporary tablespace, there’s plenty of space in your favourite tablespace, you’ve got a very boring, simple table you want to copy and partition, and no-one and nothing is...
View ArticleUpgrades
One of my maxims for Oracle performance is: “Don’t try to be too clever”. Apart from the obvious reason that no-one else may be able to understand how to modify your code if the requirements change at...
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