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 …

Just recently I came across the presentation “Bend SQL to your will with EXPLAIN” by Ligaya Turmelle via the Linked In – MySQL Speakers and Presenters group. Slides available at Slideshare. While I know a little about MySQL, I always enjoy spending time reviewing content when possible, perhaps as a good reference when asked, perhaps to glean some small point of knowledge I didn’t know, or didn’t understand completely, or had forgotten about. It’s also fun to randomly find worthwhile information.

EXPLAIN is an essential tool for MySQL developers, if you don’t know what QEP is, the listed google search link …

I generally use the following MySQL INFORMATION_SCHEMA (I_S) query to Calculate Your MySQL Database Size. This query and most others that access the MySQL INFORMATION_SCHEMA can be very slow to execute because they are not real tables and are not governed by physical data, memory buffers and indexes for example but rather internal MySQL data structures.

Mark Leith indicates in his post on innodb_stats_on_metadata that Innodb performs 8 random(ish) dives in to the index, when anybody accesses any of SHOW TABLE STATUS, SHOW INDEX, INFORMATION_SCHEMA.TABLES,INFORMATION_SCHEMA.STATISTICS for InnoDB tables. This can have an effect on performance, especially with a large number of Innodb tables, and a poor ratio of innodb_buffer_pool_size to disk data+index footprint.

What is even more …

The MySQL Innodb storage engine provides ACID compliance, the ‘I’ being isolation. There are four states defined in MySQL with the tx_isolation system variable, READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ and SERIALIZABLE.

Generally MySQL installations do not modify the default value of tx_isolation = REPEATABLE-READ, however I have seen with a number of clients the default value has been changed to READ-COMMITTED. When I attempt to identify the reason why, I have always received the same reason. Oracle uses a default transaction isolation of READ-COMMITTED. See how Oracle Manages Data …

While learning a new ORDER BY syntax recently, as a diligent architect/DBA I reviewed the documentation. What I also found in the SELECT syntax which I did not also know was the keyword DUMPFILE.

The SELECT Syntax from MySQL 5.1 Manual states:

If you use INTO DUMPFILE instead of INTO OUTFILE, MySQL writes only one row into the file, without any column or line termination and without performing any escape processing. This is useful if you want to store a BLOB value in a file.

It’s a shame there is no middle ground, where you get the features of OUTFILE (i.e. all rows), and the features of DUMPFILE (i.e. no heading)

With a discussion on identifying CPU and Memory bottlenecks achieved, let us now look at how Disk can affect MySQL performance.

One of the first steps when addressing a MySQL performance tuning problem is to perform a system audit of the physical hardware resources (CPU,Memory,Disk,Network), then identify any obvious bottlenecks in these resources.

There are several commands you can use including vmstat, iostat and sar (both part of the sysstat package) to provide information on disk I/O.

vmstat is a good place to start for an overall view for multiple resources, however for disk it only provides a total system overview.

procs -----------memory---------- ---swap-- -----io---- -system-- …

It may seem hard to believe, but I have seen DECIMAL(31,0) in action on a production server. Not just in one column, but in 15 columns just in the largest 4 tables of one schema. The column was being used to represent a integer primary or foreign key column.

In a representative production instance (one of a dozen plus distributed production database servers) the overall database footprint was decreased from ~10 GB to ~2 GB, a 78% saving. In total, 15 columns across just 4 tables were changed from DECIMAL(31,0) to INT UNSIGNED.

One single table > 5GB was reduced to under 1GB (a 81% saving). This being my record for any GB+ tables in my time working with the MySQL database.

Had this server for example had 4GB of RAM, and say 2.5GB allocated to the innodb_buffer_pool_size, this one change moved the system from requiring more consistent disk access (4x data to memory) to being able to store all data in memory. Tests showed …

Some days you learn about MySQL commands even without knowing about them. Today I wanted to check the privileges a user had because they did not have permissions to drop a view. Rather then typing SHOW GRANTS I quite by accident typed SHOW PRIVILEGES only to realize not only was it a valid command, it actually provided information that means I don’t have to go to the Privileges Provided by MySQL documentation page which I was already on.

Old dog, new trick.

mysql> show privileges;
+-------------------------+---------------------------------------+-------------------------------------------------------+
| Privilege               | Context                               | Comment                                               | …

Mark writes Now we all need the storage-engine independent test suite. I could not agree more. I have made comments about this probably as early as 4 years ago, and both before and while working for MySQL Inc.

There is however a way to do it with the current mysql-test syntax. While not ideal, it does actually work.
It took me like an hour to dig though old, old backup code, but I found it.

The Test Case:

$ cat t/engine_agnostic.test
CREATE TABLE i(id INT UNSIGNED NOT NULL);
let $ENGINE=`select variable_value from information_schema.global_variables where variable_name='STORAGE_ENGINE'`;
--replace_result $ENGINE ENGINE
SHOW CREATE TABLE i;

The Test Result:

cat r/engine_agnostic.result
CREATE TABLE i(id INT UNSIGNED NOT NULL);
SHOW CREATE TABLE i;
Table   Create Table
i       CREATE TABLE `i` (
  `id` int(10) unsigned …

I was asked this question recently “I am wanting to create a simple MySQL database consisting of 5 tables”?

While it’s easy to tell people to RTFM, the question does warrant an answer for the MySQL beginner to provide a more specific guidance as to where to start, and what to do. As a expert in MySQL it’s easy to forget how you would describe what to do. Here are my tips to getting started.

Step 1. Download the MySQL 5.1 software for your platform (e.g. Linux, Windows, Mac etc) from MySQL 5.1 Downloads. There are many different versions of MySQL, MySQL 5.1 is the current production version.

Step 2. You will need to install the MySQL software. The MySQL reference manual is the place to go, Chapter 2 describes installing MySQL. You can also download a copy of the manual in …