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Displaying posts with tag: Development with MySQL (reset)
WebScaleSQL 5.6.24 is built and ready to test

On Tuesday  Steaphan Greene  announced that all currently-pushed changes have now been rebased onto the newly-released upstream MySQL-5.6.24.

The new branch at this point is available on github.

Our .deb and .rpm builds are available in the PSCE repo as well as being browsable here

Instructions for using the repo are available for Debian …

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Handling hierarchy and travesing Social networks in MySQL with OQGraph

From time to time we detect query patterns that are not well fitted to the BTree+ structures provided by InnoDB. One such situation is when you need to traverse a hierarchy (tree) or graph structure with many nodes. Specialist databases exist for this such as Neo4J. However there exists a simple solution in the form of  OQGraph which is distributed with MariaDB and is documented here.

The OQGRAPH engine is based on an original idea by Open Query founder Arjen Lentz, and was developed in-house with Antony Curtis at Open Query.

A …

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MySQL Character encoding – part 2

In MySQL Character encoding – part 1 we stated that the myriad of ways in which character encoding can be controlled can lead to many situations where your data may not be available as expected.

UTF8 was designed on a placemat in a New Jersey diner one night in September or so 1992.

Setting MySQL Client and Server Character encoding.

Lets restart MySQL with the correct setting for our purpose, UTF8. Here we can see the setting in the MySQL configuration file, in this case /etc/mysql/my.cnf.

character-set-server = utf8

This change is then reflected in the session and global variables once the instance is restarted with the new configuration parameter.

mysql> SELECT …
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MySQL Character encoding – part 1

Breaking and unbreaking your data

Recently at FOSDEM, Maciej presented “Breaking and unbreaking your data”, a presentation about the potential problems you can incur regarding character encoding whilst working with MySQL. In short, there are a myriad of places where character encoding can be controlled, which gives ample opportunity for the system to break and for text to become unrecoverable.

The slides from the presentation are available on slideshare.

Character Encoding – MySQL DevRoom – FOSDEM 2015 from

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Tracking MySQL query history in long running transactions

Long running transactions can be problematic for OLTP workloads, particularly where we would expect most to be completed in less than a second. In some cases a transaction staying open just a few seconds can cause behaviour that is entirely unexpected, with the developers at a loss as to why a transaction remained open. There are a number of ways to find long running transactions, luckily versions of MySQL from 5.6 onwards provide some very insightful instrumentation.

Here we will use the information_schema coupled with the …

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Implementing efficient Geo IP location system in MySQL

Often application needs to know where a user is physically located. The easiest way to figure that out is by looking up their IP address in a special database. It can all be implemented in MySQL, but I often see it done inefficiently. In my post I will show how to implement a complete solution that offers great performance.

Importing Geo IP data

First you will require a database mapping network addresses to real locations. There are various resources available, but I chose the one nginx web server uses with its geoip module. GeoLite City comes in CSV format and is available for download with no charge from MaxMind.

The archive contains two files. GeoLiteCity-Blocks.csv lists all IP ranges and maps each one to the corresponding location identifier, while …

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Can MySQL use primary key values from a secondary index?

In the article about the role of a primary key, I mentioned that a secondary index in an InnoDB table consists not only of the values of its member columns, but also values of the table’s primary key are concatenated to the index. I.e. the primary key contents is part of every other index.

Assuming the following table structure:

  `id` int(10) unsigned NOT NULL,
  `user_id` int(10) unsigned NOT NULL,
  `game_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`)

Here is the visualization:

If MySQL could use in queries these implicitly added values, it would maybe allow to save some space on listing the primary key columns at the end of an index explicitly. Let’s check various cases.

Row filtering

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Joins: inner, outer, left, right

In (My)SQL, join is a means for combining records from two tables into a single set which can be either returned as is or used in another join. In order to perform the operation a join has to define the relationship between records in either table, as well as the way it will evaluate the relationship. The relationship itself is created through a set of conditions that are part of the join and usually are put inside ON clause. The rest is determined through a join type, which can either be an inner join or an outer join.

The SQL clauses that set the respective join type in a query are [INNER] JOIN and {LEFT | RIGHT} [OUTER] JOIN. As you can see the actual keywords INNER and OUTER are optional and can be omitted, however outer joins require specifying the direction – either left or right (more on that later).

Examples of queries using …

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(My)SQL mistakes. Do you use GROUP BY correctly?

Often I see a SQL problem solved incorrectly and I do not mean inefficiently. Simply incorrectly. In many cases the developer remains unaware that they aren’t getting the results they were expecting or even if a result is correct, it is only by chance, for example because the database engine was smart enough to figure out some non-sense in a query. In a few posts I will try to disclose some of the more common problems.

Aggregate with GROUP BY

Unlike many other database systems, MySQL actually permits that an aggregate query returns columns not used in the aggregation (i.e. not listed in GROUP BY clause). It could be considered as flexibility, but in practice this can easily lead to mistakes if a person that designs queries does not understand how they will be executed. For example, what values an aggregate query returns for a column that wasn’t part of the grouping key?

mysql> SELECT user_id, id, …
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Dedicated table for counters

There are a few ways to implement counters. Even though it’s not a complex feature, often I see people having problems around it. This post describes how bad implementation can impact both application and MySQL performance and how to improve it.

A customer asked me for help with performance problem they were facing. I logged into their database and found many client connections waiting for table locks. Almost all threads were stuck on one, small table called hits. What was the reason?

The problem was related to the way they developed a very simple system for counting page views they later used in some reporting. The table structure was:

*************************** 1. row ***************************
Table: hits
Create Table: CREATE TABLE `hits` (
`cnt` int(11) NOT NULL

mysql> SELECT * FROM hits;
| cnt     |
| 3823273 |

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Showing entries 1 to 10