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When is MIN(DATE) != MIN(DATE) ?

September 4, 2012 By Ernie Souhrada 5 Comments

Inspiration for this post is courtesy of a friend and former colleague of mine, Greg Youngblood, who pinged me last week with an interesting MySQL puzzle. He was running Percona Server 5.5.21 with a table structure that looks something like this:

CREATE TABLE foo (
 id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
 uid INT UNSIGNED NOT NULL,
 update_time DATETIME NOT NULL,
 ....
 INDEX `uid` (uid, update_time),
 INDEX `bar` (some_other_columns)
 ....
) ENGINE=InnoDB;

CREATE TABLE foo (

id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,

uid INT UNSIGNED NOT NULL,

update_time DATETIME NOT NULL,

....

INDEX `uid` (uid, update_time),

INDEX `bar` (some_other_columns)

....

) ENGINE=InnoDB;

When he ran this query:

SELECT MIN(update_time) FROM foo WHERE update_time IS NOT NULL AND update_time <> '0000-00-00 00:00:00';

1	SELECT MIN(update_time) FROM foo WHERE update_time IS NOT NULL AND update_time <> '0000-00-00 00:00:00';

The result came back as 2012-06-22 10:28:16. [...]

Filed Under: Insight for DBAs, Insight for Developers, MySQL

Differences between READ-COMMITTED and REPEATABLE-READ transaction isolation levels

August 28, 2012 By Justin Swanhart 3 Comments

As an instructor with Percona I’m sometimes asked about the differences between the READ COMMITTED and REPEATABLE READ transaction isolation levels. There are a few differences between READ-COMMITTED and REPEATABLE-READ, and they are all related to locking.

Filed Under: Insight for DBAs, MySQL Tagged With: ACID, InnoDB, Locking, Transaction isolation

Data compression in InnoDB for text and blob fields

May 30, 2012 By Michael Coburn 2 Comments

Have you wanted to compress only certain types of columns in a table while leaving other columns uncompressed? While working on a customer case this week I saw an interesting problem where a table had many heavily utilized TEXT fields with some read queries exceeding 500MB (!!), and stored in a 100GB table. In this [...]

Filed Under: Insight for DBAs, Insight for Developers, MySQL, Percona Software

Joining many tables in MySQL – optimizer_search_depth

April 20, 2012 By Peter Zaitsev 8 Comments

Working on customer case today I ran into interesting problem – query joining about 20 tables (thank you ORM by joining all tables connected with foreign keys just in case) which would take 5 seconds even though in the read less than 1000 rows and doing it completely in memory. The plan optimizer picked was [...]

Filed Under: Insight for DBAs, MySQL

How FLUSH TABLES WITH READ LOCK works with Innodb Tables

March 23, 2012 By Peter Zaitsev 20 Comments

Many backup tools including Percona Xtrabackup, MyLVMBackup and others use FLUSH TABLES WITH READ LOCK to temporary make MySQL read only. In many cases the period for which server has to be made read only is very short, just few seconds, yet the impact of FLUSH TABLES WITH READ LOCK can be quite large because [...]

Filed Under: Insight for DBAs, MySQL

Index Condition Pushdown in MySQL 5.6 and MariaDB 5.5 and its performance impact

March 12, 2012 By Ovais Tariq 12 Comments

I have been working with Peter in preparation for the talk comparing the optimizer enhancements in MySQL 5.6 and MariaDB 5.5. We are taking a look at and benchmarking optimizer enhancements one by one. So in the same way this blog post is aimed at a new optimizer enhancement Index Condition Pushdown (ICP). Its available [...]

Filed Under: Benchmarks, Insight for DBAs, MySQL Tagged With: Benchmarks, InnoDB, IO Performance, MariaDB, MySQL, MySQL Indexes, MySQL Performance, Optimizer

Can we improve MySQL variable handling ?

February 18, 2012 By Peter Zaitsev 8 Comments

MySQL Settings (also known as Server Variables) have interesting property. When you set variable in running server this change is not persisted in any way and server will be back to old value upon restart. MySQL also does not have option to re-read config file without restarting as some other software so approach to change [...]

Filed Under: MySQL

When EXPLAIN estimates can go wrong!

October 13, 2011 By Ovais Tariq 7 Comments

I have been working with a few customer cases and one interesting case popped up. The customer was facing a peculiar problem where the rows column in the EXPLAIN output of the query was totally off. The actual number of rows was 18 times more than the number of rows reported by MySQL in the [...]

Filed Under: MySQL

Shard-Query EC2 images available

May 11, 2011 By Justin Swanhart 2 Comments

Infobright and InnoDB AMI images are now available There are now demonstration AMI images for Shard-Query. Each image comes pre-loaded with the data used in the previous Shard-Query blog post. The data in the each image is split into 20 “shards”. This blog post will refer to an EC2 instances as a node from here [...]

Filed Under: MySQL

Should we give a MySQL Query Cache a second chance ?

April 10, 2011 By Peter Zaitsev 64 Comments

Over last few years I’ve been suggesting more people to disable Query Cache than to enable it. It can cause contention problems as well as stalls and due to coarse invalidation is not as efficient as it could be. These are however mostly due to neglect Query Cache received over almost 10 years, with very [...]

Filed Under: Insight for DBAs, MySQL

When is MIN(DATE) != MIN(DATE) ?

Differences between READ-COMMITTED and REPEATABLE-READ transaction isolation levels

Data compression in InnoDB for text and blob fields

Joining many tables in MySQL – optimizer_search_depth

How FLUSH TABLES WITH READ LOCK works with Innodb Tables

Index Condition Pushdown in MySQL 5.6 and MariaDB 5.5 and its performance impact

Can we improve MySQL variable handling ?

When EXPLAIN estimates can go wrong!

Shard-Query EC2 images available

Should we give a MySQL Query Cache a second chance ?

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