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MySQL 5.1 Plugin Development
MySQL 5.1 Plugin Development

MySQL 5.1 Plugin Development: Extend MySQL to suit your needs with this unique guide into the world of MySQL plugins

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MySQL 5.1 Plugin Development

Chapter 2. User Defined Functions

Way back in 1998, MySQL 3.21 introduced a framework that allowed users to create new SQL functions easily. It made it possible for developers to write their own functions in C/C++ and load them dynamically into the running server. The functions loaded within this framework were called User Defined Functions or UDFs.

Today not much has changed with UDFs, they are more stable and slightly more secure than they used to be, and they can be declared aggregate for use together with GROUP BY queries. However, many UDFs that worked in 1998 with MySQL 3.21.24 would still work at the time of writing in 2010, with MySQL 5.1.47.

MySQL UDFs are not part of the newer MySQL Plugin API, but there are future plans to make this happen. In the meantime, they can serve as an introduction to MySQL plugins. And sometimes UDFs can be used together with plugins to complement their functionality. In this chapter, we will cover creating User Defined Functions and write several of...

Writing UDFs


UDFs can be of two types, normal and aggregate. Normal UDFs take inputs and deliver an output just like an ordinary function in most programming languages. When run on a set of rows, they will return a result for every row. Aggregate UDFs take a group of rows, process each row, and produce a result at the end. In other words, they will return one result per group. Therefore, aggregate functions are useful for tasks such as adding up a group of values or calculating an average.

Whether MySQL considers a given UDF as an aggregate or a normal type depends on how it was installed. However, the API is somewhat different too, and we need to take care to install UDFs that use the aggregate API as aggregate and normal UDFs as normal. Otherwise they will not only fail to work correctly, but may as well crash the whole server.

Why write UDFs

There are several advantages and disadvantages to UDFs that we should be aware of. UDFs are much easier to develop than is hacking raw code into the...

A constant integer output UDF


To show the basic construction of a UDF we will start with a simple constant function that returns an integer. The output of this UDF will be the same no matter what arguments are passed to the UDF. To make the example a little more complex and to demonstrate the initid->ptr usage we will allocate a small amount of memory upon initialization to store our integer. This example will be called udf_staticexample:

#include <stdlib.h>
#include <string.h>
#include <mysql.h>

These are the standard includes needed for this example. The header mysql.h contains the structures and constants that we will use.

my_bool udf_staticexample_init(UDF_INIT *initid,
UDF_ARGS *args, char *message)
{

We are calling this UDF udf_staticexample so all functions need to be prefixed with this. We start with the udf_staticexample_init() function, which as we have seen before, prepares the UDF for execution in the context of an SQL statement.

long long *staticint =...

An integer echoing UDF


The next example is designed to show how to deal with inputs. It is similar to the previous one, but will demonstrate how to perform argument checking. As you remember, this check should happen in the initialization function, so that we can give a proper error message when there is a problem. We will call this UDF, udf_intexample, and will make it accept only one argument, which has to be an integer for the function to succeed:

#include <string.h>
#include <mysql.h>
my_bool udf_intexample_init(UDF_INIT *initid,
UDF_ARGS *args, char *message)
{

As before, we include everything needed for this example and create an initialization function with the same prefix as the function name.

if (args->arg_count != 1)
{
strcpy(message,
"udf_intexample() can only accept one argument");
return 1;
}

We want to ensure that only one argument is accepted for this UDF so we check that arg_count is 1. If it is not, we stick an error message into the message buffer and return...

A simple static text output UDF


Returning text in a MySQL UDF can be slightly more complex than dealing with numbers. MySQL gives us a memory buffer to use when returning a string but it is only 766 bytes long. This is fine for some tasks, but not big enough to hold a long string or a blob, so what we do is allocate our own buffer for this purpose. Either way, we need to tell MySQL how long the resulting string is.

The next piece of example code has two UDFs inside it, the first showing how to use the memory buffer that MySQL has given us and the second allocating its own memory. Both are simple UDFs which will just return the MySQL version number that UDF was compiled against:

#include <stdlib.h>
#include <string.h>
#include <mysql.h>
my_bool udf_textexample_init(UDF_INIT *initid,
UDF_ARGS *args, char *message)
{
initid->const_item = 1;
return 0;
}

As with previous examples we create the initialization function. We set initid->const_item to 1 here because the return...

A summing aggregate UDF


As discussed earlier in this chapter, aggregate UDFs are great for performing operations on groups of rows. Things work slightly differently here, we have a new keyword to use when installing UDFs and two extra functions to deal with individual rows and to clean up after every group of rows.

Let's write an aggregating UDF that will add up floating point numbers. We will call it udf_floatsum:

#include <stdlib.h>
#include <string.h>
#include <mysql.h>
my_bool udf_floatsum_init(UDF_INIT *initid, UDF_ARGS *args,
char *message)
{
double* float_total = malloc(sizeof(double));
*float_total = 0;
initid->ptr = (char*) float_total;

To aggregate we need an accumulator, a variable of the type double to keep the running totals as we are seeing rows one by one. Different queries invoking our UDF must have different running totals, in other words, initid->ptr is exactly what's needed. We allocate memory for one double and save a pointer to it in initid...

Further reading


There is a UDF repository run by Roland Bouman that contains various useful functions in both source and binary forms. This can be found at http://www.mysqludf.org/.

To make the UDF API even easier to use, Hartmut Holzgraefe, a MySQL Support Manager at Sun Microsystems, has created a UDF generator. This generates the source code for you based on an XML description of what the UDF should do. More information on this can be found at its PECL site at: http://pear.php.net/package/CodeGen_MySQL_UDF/

Summary


We can now see that writing a UDF is not too complex. MySQL has a lot of built-in functionality to help with the task. UDFs make a good compromise between easy and limited Stored Procedures and the ultimate power and complexity of modifying the source code of the MySQL server.

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Key benefits

  • A practical guide with working examples explained line by line
  • Add new functions to MySQL with User Defined Functions
  • Export information via SQL using the INFORMATION_SCHEMA plugins
  • Search within PDFs, MP3s, and images; offset user typing errors with fulltext parser plugins
  • Access your custom data formats with a storage engine plugin

Description

MySQL has introduced a Plugin API with its latest version – a robust, powerful, and easy way of extending the server functionality with loadable modules on the fly. But until now anyone wishing to develop a plugin would almost certainly need to dig into the MySQL source code and search the Web for missing bits of the information.This is the first book on the MySQL Plugin API. Written together with one of the Plugin API primary architects, it contains all the details you need to build a plugin. It shows what a plugin should contain and how to compile, install, and package it. Every chapter illustrates the material with thoroughly explained source code examples.Starting from the basic features, common to all plugin types, and the structure of the plugin framework, this book will guide you through the different plugin types, from simple examples to advanced ones. Server monitoring, full-text search in JPEG comments, typo-tolerant searches, getting the list of all user variables, system usage statistics, or a complete storage engine with indexes – these and other plugins are developed in different chapters of this book, demonstrating the power and versatility of the MySQL Plugin API and explaining the intricate details of MySQL Plugin programming.

Who is this book for?

If you are a MySQL application developer or a programmer who wants to interface with MySQL server, this book is for you. Basic knowledge of C/C++, SQL, and MySQL in particular is assumed in the book, but no deep familiarity with any of the topics is required.

What you will learn

  • Build and install a plugin, and learn how to package it for distributing separately from MySQL
  • Add new functions to MySQL using the UDF interface
  • Declare new MySQL status and configuration variables from a plugin, and use Daemon plugins for a simple push style server monitoring
  • Add a new table to the INFORMATION_SCHEMA that shows anything you like with an INFORMATION_SCHEMA plugin
  • Use fulltext parser plugins to tolerate user errors, and search for text in binary data formats, for example, JPEG, or text that is stored outside the database
  • Store and retrieve rows and convert your custom data format to what MySQL understands using the storage engine plugin
  • Create a parser that indexes comments from JPEG files using the libexif library
  • Understand how the indexing API works, and how to work around the incompatibilities between your indexing engine and MySQL expectations
  • Learn about new features of the Plugin API, available beyond MySQL 5.1
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Publication date, Length, Edition, Language, ISBN-13
Publication date : Aug 26, 2010
Length: 288 pages
Edition : 1st
Language : English
ISBN-13 : 9781849510608
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Category :
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Publication date : Aug 26, 2010
Length: 288 pages
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ISBN-13 : 9781849510608
Vendor :
MySQL
Category :
Tools :

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Table of Contents

10 Chapters
Compiling and Using MySQL Plugins Chevron down icon Chevron up icon
User Defined Functions Chevron down icon Chevron up icon
Daemon Plugins Chevron down icon Chevron up icon
Information Schema Plugins Chevron down icon Chevron up icon
Advanced Information Schema Plugins Chevron down icon Chevron up icon
Full-text Parser Plugins Chevron down icon Chevron up icon
Practical Full-text Parsers Chevron down icon Chevron up icon
Storage Engine Plugins Chevron down icon Chevron up icon
HTML Storage Engine—Reads and Writes Chevron down icon Chevron up icon
TOCAB Storage Engine — Implementing Indexes Chevron down icon Chevron up icon

Customer reviews

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Giuseppe Maxia Nov 03, 2010
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Highly recommended. If you want to develop MySQL extensions, buy this book. It's a must, written by two expert professionals who probably know more than anyone else on this matter. The book is full of practical examples explained with the theoretical information necessary to make it stick.
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Mark Callaghan Aug 05, 2011
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I read this book last year and was impressed. It is written by experts so there is a lot of valuable information. But they also took the time, a huge amount of time, to write this so that it is easy to read. I don't know the editor but will thank him/her as well. [...]
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