Pythian Group

Today’s trivial MySQL system variable:

old_alter_table

The interesting bit is that this is a system variable, and shows up in SHOW GLOBAL VARIABLES, but is not documented on the Server System Variables manual page.

Instead, it is documented on the manual page for Server options.

Unfortunately, that documentation is very sparse. It tells us:

old-alter-table is an option that can be set in an option file (such as /etc/my.cnf)

old_alter_table is the name of the variable.

And….that’s it. It is neither a system nor status variable, there is no scope, and no entry for whether or not it is dynamic. These last 2 are baffling, though they show up in other variables in the matrix on the “server options” manual page. The scope is either session or global; there is no NULL — the scope may not be *relevant*, but it still exists.

As well, either the variable can be settable on commandline, or not. There is no meaning to a NULL value in that column.

The most egregious issue is that there is no documentation whatsoever about what the variable does. What happens if I set old_alter_table to ON?

(My guess is that it’s a placeholder for the behavior of ALTER TABLE in 5.0 and earlier, perhaps it disables the use of ALTER TABLE ONLINE? My second guess is that whatever the functionality, it is not implemented yet, similar to date_format).

Last week I had to confront one of those situations where you can’t really tell what is going on with a piece of software, and the final conclusion would sound completely crazy if postulated as the initial hypothesis. The regular MySQL commands and utilities fall short in these cases, so I had to resort to the three tools reviewed in this article.

The problem we were diagnosing was this: at some point in time, a number of queries that use to take less than one minute to execute, started to take between five to 15 minutes. We needed to get an insight into what was going on inside the MySQL server.

MySQL Tuner

At some point in a long diagnosis process, MySQL’s SHOW [GLOBAL] VARIABLES and SHOW [GLOBAL] STATUS are nothing more than a long list of numbers. Going through a team mate’s notes on another issue, I came across MySQL Tuner. This is an extremely simple tool that takes the information from the SHOW statements mentioned above and combines it in a useful way. Sample run:

Read the rest of this entry . . .