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Here’s a list of reasons why SELECT * is bad for SQL performance, assuming that your application doesn’t actually need all the columns. When I write production code, I explicitly specify the columns of interest in the select-list (projection), not only for performance reasons, but also for application reliability reasons. For example, will your application’s data processing code suddenly break whe

Update: I wrote a follow-up article about adding Loop Counters and Row Counts to SQL Plan FlameGraphs. I also show some old examples of “Bookshelf Graph” visualizations there. Check it out after reading this post first. Introduction Brendan Gregg invented and popularized a way to profile & visualize program response time by sampling stack traces and using his FlameGraph concept & tools. This techn

Black Friday sale of my full-week training videos: check it out here! [ part 1 | part 2 | part 3 ] In the previous article I explained that the main requirement for high-speed in-memory data scanning is column-oriented storage format for in-memory data. SIMD instruction processing is just icing on the cake. Let’s dig deeper. This is a long post, you’ve been warned. Test Environment I will cover fu

Peeking into Linux kernel-land using /proc filesystem for quick’n’dirty troubleshooting This blog entry is about modern Linuxes. In other words RHEL6 equivalents with 2.6.3x kernels and not the ancient RHEL5 with 2.6.18 kernel (wtf?!), which is the most common in enterprises unfortunately. And no, I’m not going to use kernel debuggers or SystemTap scripts here, just plain old “cat /proc/PID/xyz” c

A lot of people have asked me whether there’s some sort of index or “table of contents” of my TPT scripts (there’s over 1000 in my TPT script repo right now). I have planned to create such index for years, but never got to it. I probably never will :) So a good way to extract the descriptions of some scripts is to just grep for Purpose: in the scripts directory: grep -i Purpose: ash/*.sql awr/*.sq

Advanced Oracle Troubleshooting Guide – Part 10: Index unique scan doing multiblock reads?! When you troubleshoot an Oracle (performance) problem you usually want to find out what are the problem sessions doing (wrong) and then why are they doing it. The “what” question is usually answered using some wait interface based method – like ASH or SQL*Trace, which both add plenty of extra details to the