In writing a recommendation for our Web development team on how to use MySQL, I came up with the following list, which I want to share: What kind of queries are bad for MySQL?

• Any query is bad. Send a query only if you must. (Hint: use caching like memcache or redis)
• Queries that examine many rows are bad. Try instead to use…
  

      SELECT col1 FROM table1 WHERE primary_key_column=SOMETHING

  
  . Or at least
  

  secondary_key_column=SOMETHING

  
  . If it is still not possible, try to make the query examine the least amount of rows possible (zero is ideal, as we come to the first case here)
• Queries with JOINS are bad. Try to denormalize the table to avoid JOINS. Example: original query
  

      SELECT t2.value FROM t2 JOIN t1 ON (t1.id=t2.tid) WHERE t1.orderdate=NOW()

  
  . This can be denormalized by copying the column orderdate from table …

MySQL 5.6 introduces a new feature – microseconds resolution in some temporal types. As of 5.6.4 TIME, TIMESTAMP and DATETIME can have a fractional part. To create a field with subseconds you can specify precision in brackets: TIME(3), DATETIME(6) etc.

Obviously, the new feature requires the format change. All three types may now have a tail with a fractional part. The tail may be one, two or three bytes long and can store up to six digits after the point.

The non-fractional part has changed too. Thus, DATETIME uses only five bytes comparing to eight in previous versions.

As of revision 79 the …

Dolphin and Elephant: an Introduction

This post is intended for MySQL DBAs or Sysadmins who need to start using Apache Hadoop and want to integrate those 2 solutions. In this post I will cover some basic information about the Hadoop, focusing on Hive as well as MySQL and Hadoop/Hive integration.

First of all, if you were dealing with MySQL or any other relational database most of your professional life (like I was), Hadoop may look different. Very different. Apparently, Hadoop is the opposite to any relational database. Unlike the database where we have a set of tables and indexes, Hadoop works with a set of text files. And… there are no indexes at all. And yes, this may be shocking, but all scans are sequential (full “table” scans in MySQL terms).

So, when does Hadoop makes sense?

First, Hadoop is great if you need to …

For the same customer I am exploring ZFS for backups, the twin server is using regular LVM and XFS. On this twin, I have setup mylvmbackup for a more conservative backup approach. I quickly found some odd behaviors, the backup was taking much longer than what I was expecting. It is not the first time I saw that, but here it was obvious. So I recorded some metrics, bi from vmstat and percent of cow space used from lvs during a backup. Cow space is the Copy On Write buffer used by LVM to record the modified pages like they were at the beginning of the snapshot. Upon reads, LVM must scan the list to verify that there’s no newer version. Here’s the other details about the backup:

• Filesystem: 2TB, xfs
• Snapsize: 60GB
• Amount to backup: ~600GB
• Backup tool: mylvmbackup
• Compressor: pbzip2

…

Among many of the improvements you can enjoy in MySQL 5.6, there is one that addresses a huge operational problem that most DBAs and System Administrators encounter in their life: schema changes.

While it is usually not a problem for small tables or those in early stages of product life cycle, schema changes become a huge pain once your tables get a significant amount of data. Planning for maintenance is becoming more and more difficult, and your worldwide users want the service to be up and running 24/7, while on the other hand, your developers desire to introduce schema changes every week.

PITA

But what is the real problem here? Let me illustrate very typical case:

Session1> ALTER TABLE revision ADD COLUMN mycol tinyint;
Query OK, 1611193 rows affected (1 min 5.74 sec)
Records: 1611193  Duplicates: 0 …

One very interesting feature, “Compact Backup,” is introduced in Percona XtraBackup 2.1. You can run “compact backups” with the  –compact option, which is very useful for those who have limited disk space to keep the database backup. Now let’s first understand how it works. When we are using –compact option with Innobackupex, it will omit the secondary index pages. This will make the backups more compact and this way they will take less space on disk but the downside of this is, the backup prepare process will take longer time because those secondary index pages will be recreated while preparing the backup. Here, we need to consider couple of things before implement …

Statement-based or row-based, or mixed?  We’ve all seen this discussed at length so I’m not trying to rehash tired arguments.  At a high level, the difference is simple:

1. Statement based replication (SBR) replicates the SQL statements to the slave to be replayed
2. Row based replication (RBR) replicates the actual rows changed to the slave to be replayed
3. Mixed mode uses RBR in the event of a non-deterministic statement, otherwise uses SBR

Recently, I worked with a client to optimize their use of pt-online-schema-change and keep replication delay to a minimum.  We found that using RBR in conjunction with a smaller chunk-time was the best result in their environment due to reduced IO on the slave, but I wanted to recreate the test locally as well to see how it looked in the generic sense (sysbench for data/load).

Here was my local setup:

…

There are different articles on how to setup MySQL with SSL but it’s sometimes difficult to end up with a good simple one. Usually, setting up MySQL SSL is not really a smooth process due to such factors like “it’s not your day”, something is broken apparently or the documentation lies I am going to provide the brief instructions on how to setup MySQL with SSL, SSL replication and how to establish secure connections from the console and scripts showing the working examples.

Quick links:

• Setup SSL on MySQL
• Establish secure connection from console
• Setup SSL replication
• Establish …

Recently, I was working with a client that asked about using Percona Xtrabackup to take a snapshot of a particular MySQL schema and then reload it with a different schema name on the same server.  It caught me off guard because I’d never really thought about it – typically, I’ve used Xtrabackup simply to clone a server (for replication or migration) or migrate an existing schema to a new server.  However, given the import/export functionality of Xtrabackup combined with Percona Server (Exporting and Importing Tables), it did seem …

Thanks to everyone who was in attendance on 05 June 2013 for my “Choosing a MySQL HA Solution” webinar. If you weren’t able to make it but are interested in listening to the presentation, it’s currently up and available for viewing over at percona.com.

My apologies if we weren’t able to get to your question during the initial session, so I’ll address those lingering questions in this post, along with providing a bit more detail on some of the questions that I did cover during the session.

Q: What is the reason that I recommended DRBD be used only on physical hardware and not on virtual machines?
A: I covered this a bit during the session, but to provide a bit more commentary. There …