OUTER JOIN queries in SQL are susceptible to two very subtle bugs that I’ve observed a number of times in the real world. Daniel and I have been hammering out ways to automatically detect queries that suffer from these bugs, in a relatively new Maatkit tool called mk-query-advisor. It’s part of our series of advisor tools for MySQL. I wrote a blog post about it a while ago. Automated analysis of bad query patterns is a good thing to write tools to do, because catching buggy queries is hard work if you do it manually.

Let’s dive right in and analyze these subtle bugs. Warning: if you don’t understand how SQL handles NULL, you’re not going to understand the following. Many people have a hard time with NULL, which is why these bugs are so hard to …

In October 2008, Baron posted How to Check MySQL Replication Integrity Continually. Here at Pythian we have developed a method based on that post, and added “verifying that masters and slaves are in sync” to our standard battery of tests.

We call it “Continual replication sync checking”. This article will explain how it works, how to test and make the procedure non-blocking, benchmarks from the real world, issues we encountered along the way, and …

For those of you who miss what Maatkit’s mk-audit tool (now retired) gave you, there’s a pair of tools in Aspersa that more than replaces it. I wrote previously about the summary tool. I don’t think I have mentioned the mysql-summary tool. It has been under development for a while, and at this point it has quite a lot of functionality. You can see a sample of the output on its wiki page.

Related posts:

1. Apsersa’s summary tool supports Adaptec and MegaRAID …

The Maatkit article on Wikipedia was removed some time ago, after being deemed not notable. I believe this is no longer the case. It’s hard to find a credible book published on MySQL in the last few years that doesn’t mention Maatkit, there’s quite a bit of blogging about it from MySQL experts and prominent community members, and the toolkit is certainly in wide use — it’s important enough that notable companies are supporting its development. It’s available through every major Unix-like operating system’s package repository. On Debian, it’s actually part of the mysql-client package, so if you install MySQL, you automatically get Maatkit too. I believe it’s probably the second most important set of MySQL command-line tools; the most …

The Maatkit toolkit for MySQL has a lot of functionality that’s common across the tools. It’s not a good idea to document this in each tool’s man page, of course. So there is an overall maatkit man page. It explains concepts such as configuration file syntax. This and all the other Maatkit man pages are online.

Related posts:

1. How PostgreSQL protects against partial page writes and data corruption
2. Writing a book about Maatkit
3. …

The Maatkit tools provide a suite of additional MySQL commands. There is one command I use constantly and that is mk-query-digest.

Unfortunately the documentation does leave a lot to be desired for usability. While throughout, it is a man page and not a user guide. Several of us have discussed writing better documentation however it’s always a matter of time. I have however learned a number of tips and I’d like to share them in smaller digests.

The first is showing additional display. Maatkit works on truncating per line output to a reasonable length of 73 characters?

One of those lines is the list of hosts that connected to MySQL for a query, for example.

# Hosts                  4 192.168.40... (2), 192.168.40... (2)... 2 more
# Hosts                  3 99.99.245.14 (12), …

While I use this tcpdump command for MySQL query analysis with mk-query-digest, I found recently that it didn’t work on FreeBSD

$ tcpdump -i bge0 port 3306 -s 65535 -x -n -q -tttt -c 5
tcpdump: syntax error

It left me perplexed and reading the man page seemed to indicate my options were valid. I tried a few variances just to be sure without success.

$ tcpdump -i bge0 -c 5 port 3306 -x
tcpdump: syntax error
$ tcpdump -i bge0 -c 5 port 3306 -q
tcpdump: syntax error
$ tcpdump -i bge0 -c 5 port 3306 -tttt
tcpdump: syntax error

The solution was actually quite simple in the end, it had nothing to do with the commands, it had everything to do with the order of them. Placing port as the last option solved the problem.

$ tcpdump -i bge0 -s 65535 -x -n -q -tttt -c 5  port 3306

$ uname -a
FreeBSD …

I’m surprised, and flattered, to see that people still use, write and recommend mysqlsla, mysqlreport and–most surprisingly–mysqlsniffer. In truth, however, I consider all the original Hack MySQL tools as retired. Maatkit consumes the majority of my development time and provides better replacements for all the Hack MySQL tools. The mk tools are better because–most importantly–they’re tested, their code is more robust, and they benefit from the collected knowledge and experience of the community’s top minds (whereas the Hack MySQL tools are brain-children of only my knowledge and experience circa several years ago).

Thus I created a new tools page where I list and briefly profile free, open-source MySQL tools. As the intro paragraph states, MySQL Forge does this, too, but imho the forge is a dense jungle in which it is difficult to discern the useful bits from the …

Daniel changed mk-query-digest to use much less memory. It parsed and aggregated a 2GB MySQL slow query log file in a few dozen megabytes of memory for me yesterday. Thanks to Facebook for sponsoring this work.

Related posts:

1. mk-query-digest now supports Postgres logs
2. Learn about mk-query-digest at PgEast 2010
3. Slides from my session on mk-query-digest at PgEast 2010
4. …

If you’re new to the MySQL DBA role, you’ll be excited to learn about the Maatkit toolset.  It provides a whole host of valuable functionality and fills many of the DBAs day-to-day needs.

DatabaseJournal – Wonders of Maatkit