Posts Categorized: Pythian

MySQL: Tuning filesorts and temporary tables

Filesorts and temp tables are a necessary evil in MySQL, used when MySQL must sort the data before returning the output to the user. They are the most common issue with slow queries in MySQL, the main reason being that if the output is too large, you can kiss goodbye in-memory performance, and say hello to disk access.

MySQL Can’t Use Index With Uncorrelated IN Subquery

Today is the first time I had to look at MySQL performance. Tiny database as web application back-end was having significant performance issues with spikes of CPU workload. After identifying problematic queries, I found a pile of statements using IN subqueries. I asked around and our MySQL experts assured me that this is one of the minor and not so disturbing issues, in fact. I can’t imagine what those disturbing issue are. I guess Oracle XE does have some advantages over MySQL for small installations.

Welcome Doug!

Doug Burns is finally here in Ottawa. You must see his smile in the airport – somehow he managed to read my comment and already expected me while I thought it would be surprise.

Twitter, Pythian Style… also Oracle’s SYSTEM_PRIVILEGE_MAP

To the subject at hand (psst, there will be some technical stuff in the end)… A few days ago I came across Twitter. I liked that idea very much and even tried it for one evening. I don’t think Twitter fits my lifestyle, but I recognized a very familiar concept. Indeed, it reminds me of one feature of our Pythian Support Track. Time for a bit of technical stuff…since we are mostly talking about databases on this blog, here is a view in Oracle I found very useful one day: SYSTEM_PRIVILEGE_MAP.

Oracle Gotcha: Months_between 31-days-per-month assumption

Over-the-Top Tales from the Trenches:: Bringing order to the chaos of everyday DBA life.

Do you want to know a secret? Understanding it will prevent pain and gnashing of teeth, and also leave your face comfortably free of egg. Here it is: Some Oracle functions assume there are 31 days for each month of the year! The moral of the story: be careful when using MONTHS_BETWEEN for fractional dates.

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