Just how effective is the Query Cache on performance? If you are not comfortable reading the MySQL code you can cheat very easily with the SHOW PROFILE command. This demonstration will show you at a high level the relative impact without and with the Query Cache. First let us confirm the Query Cache is not used.

mysql> SHOW GLOBAL VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 0       |
| query_cache_type             | ON      |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

We now enable profiling.

mysql> SET PROFILING=1;

We run our …

Maximize your strengths, minimize your weaknesses.

You can apply this approach to many things in life, I apply it to describing and using MySQL the product, and it’s components. The Query Cache like many features in MySQL, and indeed features in many different RDBMS products (don’t get me started on Oracle *features*) have relative benefits. In one context it can be seen as ineffective, or even detrimental to your performance, however it’s course grain nature makes it both trivial to disable dynamically (SET GLOBAL query_cache_size=0;), and also easy to get basic statistics on current performance (SHOW GLOBAL STATUS LIKE ‘QCache%’;) to determine effectiveness and action appropriately.

The Query Cache is course grained, that is it is rather simple/dumb in nature. When you understand the path of execution of a query within the MySQL kernel you learn a few key things.

• When enabled, by default the Query …

MySQL uses locks for concurrency control. Whenever a client/thread acquires a lock, it will have exclusive access to that table or row (depending on the granularity of the lock). Other clients however, will be prevented from writing and possibly reading to/from the locked resource. The two main existing locks are:

READ LOCK – A read lock will allow the other clients to read from the locked resource but not write to it.
WRITE LOCK – a write lock will prevent the other clients from reading or writing to the locked resource.

Also, different storage engines have different lock granularity. For example, MyISAM will lock the whole table while InnoDB will only lock the rows it needs.

Let’s try this out using a MyISAM table. We open two concurrent sessions and in the first lock the City table from the world database (get it …

In two words: online operations. In a paragraph: Forget partitioning, row-based replication and events. The big reasons most people are going to salivate over 5.1, and probably start plans to upgrade now, are the online operations:

• online ALTER TABLE for column rename, column default value change, and adding values to the end of an ENUM/SET
• Online, table-based logging. No more need to restart your server to enable or change the general or slow query logs. You can have the standard file-based output or choose a table format…which you can query.

(more…)

A while ago I wrote about MySQL 5.1 information schema plug-ins.

At the time, I wrote a plug-in to report the contents of the query cache, but for all kinds of reasons, I never found the time to write a decent article about it, nor to release the code.

I am not sure if I'll ever find the time to write that article, but I just tidied up the code, and installed it in a new MySQL 5.1.26-rc server. It seems to work there, so I put the code up on the web.

Inside the source file, there's instructions to build and deploy it. If all goes well, you can do …

Day two of the conference was a little disappointing, as far as sessions went. There were several time blocks where I simply wasn’t interested in any of the sessions. Instead, I went to the expo hall and tried to pry straight answers out of sly salespeople. Here’s what I attended.

Paying It Forward: Harnessing the MySQL Contributory Resources

This was a talk focused on how MySQL has made it possible for community members to contribute to MySQL. There was quite a bit of talk about IRC channels, mailing lists, and the like. However, the talk gave short shrift to how MySQL plans to become truly open source (in terms of its development model, not its license). I think there was basically nothing to talk about there. I had a good conversation about some of my concerns with the speaker and some others from MySQL right afterwards.

There was basically nobody there — I didn’t count, but I’d say maybe 10 or 12 people. I …

I'll be attending the 2008 MySQL Conference and Expo again this year, and I'm looking forward to hearing some great sessions, meeting new and old friends, and giving sessions myself. As a proposal reviewer, I looked at and voted on 250+ proposals for sessions and tutorials for this conference. There are going to be some great sessions and tutorials.