Difference between DISTINCT and GROUP BY

Today we had an interesting situation where the same query was executed significantly slower when it was written with GROUP BY instead of DISTINCT and I saw many people still had the assumption that these two types of queries are actually equivalent which is simply not true. Although DISTINCT queries … Continue reading

MySQL 5.6 mysql_install_db script problem

We’re always testing the latest versions of MySQL with most of the environments to make sure that we can find the critical issues before it goes to production. This wasn’t different with the 5.6 MySQL neither. We already started to play with this version in the summer. The first news … Continue reading

MySQL Benchmark – updates by primary vs secondary keys

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(Note: when I’m talking about MySQL I usually assume InnoDB storage engine. Any other case I explicitly tell this is MyISAM or Memory etc.)

I’ve heared an interesting aproach of using Master-slave replication in MySQL.

Thesis

So the theory was that since updates by primary keys are fast and by … Continue reading

Sleeping connections

Why is it so important to close connections to databases if there’s no explicit need to keep them open (which usually the case)?

Symptoms

  • Icinga reports high usage of allowed connections (>90%) on master MySQL server.
  • No running queries or any specific in processlist. All the connections are sleeping … Continue reading

Unindexed queries can be really expensive

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The story happened with a webshop application running on Amazon EC2 microinstances. Actually on two instance. Amazon business model is basically simple, they ask money for only three things: Cpu time, IOPS and network traffic. Everybody (including me) thinks for the first time network traffic will be the bottleneck until … Continue reading