There’s a really important difference between a unique index (MySQL’s answer to a “unique constraint”) and a primary key in MySQL. Please take a look at this:

CREATE TABLE `t` (
  `a` int,
  `b` int,
  `c` int,
  UNIQUE KEY `a` (`a`,`b`)
)

The combination of columns a, b should uniquely identify any tuple in the table, right?

select * from t;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    2 |    3 | 
| NULL | NULL |    1 | 
| NULL | NULL |    1 | 
| NULL | NULL |    1 | 
+------+------+------+

Wrong. Our arch-enemy NULL messes things up again:

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does …

A lot of times I’m looking at several time-series samples of numbers embedded in free-form text, and I want to know how the numbers change over time. For example, two samples of SHOW INNODB STATUS piped through grep wait might contain the following:

Mutex spin waits 0, rounds 143359179688, OS waits 634106844
RW-shared spins 1224152309, OS waits 38278807; RW-excl spins 2432166425, OS waits 35264871
Mutex spin waits 0, rounds 143386303439, OS waits 634292093
RW-shared spins 1224197048, OS waits 38281423; RW-excl spins 2432347936, OS waits 35271423

How much have the numbers changed in the second sample? My head is too lazy to do that math. So Daniel Nichter and I whipped up Yet Another Snippet to self-discover patterns of text and numbers, and compare each line against the previous line that matches the same pattern. Let’s fetch it:

wget  …

I’ve been thinking recently about the failure scenarios of MySQL replication clusters, such as master-master pairs or master-master-with-slaves. There are a few tools that are designed to help manage failover and load balancing in such clusters, by moving virtual IP addresses around. The ones I’m familiar with don’t always do the right thing when an irregularity is detected. I’ve been debating what the best way to do replication clustering with automatic failover really is.

I’d like to hear your thoughts on the following question: what types of scenarios require what kind of response from such a tool?

I can think of a number of failures. Let me give just a few simple examples in a master-master pair:

Problem: Query overload on the writable master makes mysqld unresponsive

Do nothing. Moving the queries to another server will cause cascading failures.

Problem: The …

A short time ago in a galaxy nearby, Domas Mituzas wrote about contention profiling with GDB stack traces. Mark Callaghan found the technique useful, and contributed an awk script (in the comments) to aggregate stack traces and identify which things are blocking most threads. I’ve used it myself a time or five. But I’ve found myself wanting it to be fancier, for various reasons. So I wrote a little utility that can aggregate and pretty-print backtraces. It can handle unresolved symbols, and aggregate by only the first N lines of the stack trace. Here’s an example of a mysqld instance that’s really, really frozen up:

bt-aggregate -4 samples/backtrace.txt | head -n12
2396 threads with the following stack trace:
        #0  0x00000035e7c0a4b6 in …

I’m going to present on Maatkit at the CPOSC conference in central Pennsylvania on Saturday, October 17th 2009. I’ll give an overview of the toolkit, which is no longer an easy task in a single session. I see a number of other interesting sessions have been accepted. It looks like it’ll be a good conference.

Related posts:

1. Speaking at EdUI Conference 2009 I’m
2. Learn about Maatkit at the MySQL Conference I’m
3. Presentation uploaded …

Yesterday was a good vacation day at my sister's house. As usual with me, I don't seem able to have a completely computer-free day. I did some work tasks, and then turned my attention to my sister's network problems. She has a Mac, which is wired, and several Windows laptops. The windows laptops weren't able to connect to the Verizon-supplied DSL modem and router in-one. It was set up for WEP (64-bit). I tried numerous times to get the windows laptops working and found finally that that using 128-bit encryption WEP solved the problem. Except for one computer. It was an old Dell laptop with Windows. She wants this to be her daughter's (my 11 year-old niece) computer. This laptop was running extremely slow and could not connect to the Verizon router, even when not encrypted. This particular laptop also uses a PCI Linksys wireless card, so perhaps it's out of date enough to not work with the Verizon router.

My sister asked me if I could …

Beginning Database Design: From Novice to Professional

Beginning Database Design: From Novice to Professional. By Clare Churcher, Apress, 2007. Page count: 230 pages. (Here’s a link to the publisher’s site.)

My wife bought a copy of this book, and recently I took it off her bookshelf to give it a read myself.

I found the book very lucid and readable. The author does not drag us through a bunch of formalisms, nor does she attempt to force the book to be readable through the use of comics, pop-culture references, or other artificial devices. Instead, she draws on her real-life experience helping people design databases, and presents several examples that …

Looking at twitter #songsincode (just search on #songsincode tag), it appears a large chunk of geeky/nerdy world has come to a halt while spending the day expression song titles in code. So far we’ve seen most programming languages as well as CSS and SQL come by. I think it’s a nice example of how “the collective” can become very creative. My favourite SQL ones so far (by @john_chr): SELECT * FROM walk WHERE gait LIKE '%EGYPTIAN%'

Update: a good friend of mine, Steve Thorne (@Jerub), wanted to set up a site for this, so we hacked one up: SongsInCodeDB.

The program is very rich. There will be several tracks in the main event and in the associated conferences. If …

You probably know that it’s possible to set configuration variables to log queries that don’t use indexes to the slow query log in MySQL. This is a good way to find tables that might need indexes.

But what if the slow query log isn’t enabled and you are using (or consulting on) MySQL 5.0 or earlier, where it can’t be enabled on the fly unless you’re using a patched server such as Percona’s enhanced builds? You can still capture these queries.

The key is knowing what it really means for a query to “not use an index.” There are two conditions that trigger this — not using an index at all, or not using a “good” index. Both of these set a bit. If either bit is set, the query is captured by the filter and logged. Both of these bits also …