27.3. Adding New Functions to MySQL
There are two ways to add new functions to MySQL:
You can add functions through the user-defined function (UDF)
interface. User-defined functions are compiled as object files
and then added to and removed from the server dynamically
using the CREATE FUNCTION and DROP
FUNCTION statements. See
Section 27.3.2, “CREATE FUNCTION Syntax”.
You can add functions as native (built-in) MySQL functions.
Native functions are compiled into the
mysqld server and become available on a
permanent basis.
Each method has advantages and disadvantages:
If you write user-defined functions, you must install object
files in addition to the server itself. If you compile your
function into the server, you don't need to do that.
Native functions require you to modify a source distribution.
UDFs do not. You can add UDFs to a binary MySQL distribution.
No access to MySQL source is necessary.
If you upgrade your MySQL distribution, you can continue to
use your previously installed UDFs, unless you upgrade to a
newer version for which the UDF interface changes. For native
functions, you must repeat your modifications each time you
upgrade.
Whichever method you use to add new functions, they can be invoked
in SQL statements just like native functions such as
ABS() or SOUNDEX().
Another way to add functions is by creating stored functions.
These are written using SQL statements rather than by compiling
object code. The syntax for writing stored functions is described
in Chapter 19, Stored Procedures and Functions.
The following sections describe features of the UDF interface,
provide instructions for writing UDFs, discuss security
precautions that MySQL takes to prevent UDF misuse, and describe
how to add native mySQL functions.
For example source code that illustrates how to write UDFs, take a
look at the sql/udf_example.cc file that is
provided in MySQL source distributions.
