Another reason to avoid RDS

My list of reasons for never using or recommending Amazon’s MySQL RDS service grows every time I experience problems with customers. This was an interesting and still unresolved issue.

ERROR 126 (HY000): Incorrect key file for table '/rdsdbdata/tmp/#sql_5b7_1.MYI'; try to repair it

You may see this is a MyISAM table. The MySQL database is version 5.5, all InnoDB tables and is very small 100MB in total size.
What is happening is that MySQL is generating a temporary table, and this table is being written to disk. I am unable to change the code to improve the query causing this disk I/O.

What I can not understand and have no ability to diagnose is why this error occurs sometimes and generally when the database is under additional system load. With RDS you have no visibility of the server running the production database. While you have SQL access, an API for managing MySQL configuration options (I also add not all MySQL variables), and limited system statistics via a graphical interface, all information about the system performance, disk configuration etc is hidden and not accessible. This is a frustrating limitation of using RDS.

NOTE: While I cannot recommend RDS, I am very happy with AWS EC2 services when correctly configured. For a cloud based MySQL solution I would definitely recommend greater control over your MySQL database using EC2 and EBS.

Tagged with: Amazon Web Services (AWS) Databases EC2 MySQL RDS

Producing Alternative Means statistics with SQL

Producing Alternative Means statistics with SQL

MySQL’s built-in AVG() computes the arithmetic mean — the sum divided by the count. That is the right default for many questions, but it is not always the right measure of central tendency.

Extending MySQL Capabilities with UDFs, Plugins and Components

Extending MySQL Capabilities with UDFs, Plugins and Components

MySQL offers three different approaches to extending the SQL capabilities with the default product you download and install. These are: User Defined Function (UDF) MySQL Manual MySQL Plugin MySQL Manual MySQL Component MySQL Manual For the purposes of this post I will be using the current LTS version MySQL 8.

Producing One-Sample Z-Test statistics with SQL

Producing One-Sample Z-Test statistics with SQL

The one-sample Z-test determines whether a sample mean differs significantly from a known population mean when the population standard deviation is also known. It is the appropriate test when the population parameters are established — quality control benchmarks, national averages, long-run process measurements — and you want to evaluate whether a new sample is consistent with them.