To add a new native MySQL function, use the procedure described here, which requires that you use a source distribution. You cannot add native functions to a binary distribution because it is necessary to modify MySQL source code and compile MySQL from the modified source. If you migrate to another version of MySQL (for example, when a new version is released), you must repeat the procedure with the new version.
If the new native function will be referred to in statements that will be replicated to slave servers, you must ensure that every slave server also has the function available. Otherwise, replication will fail on the slaves when they attempt to invoke the function.
To add a new native function, follow these steps to modify
source files in the
sql directory. For
MySQL 5.1, the first two steps apply only as of 5.1.13. For
older versions, see the instructions in the corresponding
section of the MySQL 5.0 manual.
Create a subclass for the function in
If the function takes a fixed number of arguments,
create a subclass of
depending on whether the function takes zero, one, two,
or three arguments. For examples, see the
If the function takes a variable number of arguments,
create a subclass of
Create_native_func. For an example,
To provide a name by which the function can be referred to
in SQL statements, register the name in
item_create.cc by adding a line to this
static Native_func_registry func_array
You can register several names for the same function. For
example, see the lines for
"LOWER", which are aliases for
item_func.h, declare a class
Item_str_func, depending on whether your
function returns a number or a string.
item_func.cc, add one of the
following declarations, depending on whether you are
defining a numeric or string function:
double Item_func_newname::val() longlong Item_func_newname::val_int() String *Item_func_newname::Str(String *str)
If you inherit your object from any of the standard items
Item_num_func), you probably only
have to define one of these functions and let the parent
object take care of the other functions. For example, the
Item_str_func class defines a
val() function that executes
atof() on the value returned by
If the function is nondeterministic, include the following statement in the item constructor to indicate that function results should not be cached:
A function is nondeterministic if, given fixed values for its arguments, it can return different results for different invocations.
You should probably also define the following object function:
This function should at least calculate
max_length based on the given arguments.
max_length is the maximum number of
characters the function may return. This function should
maybe_null = 0 if the main
function can't return a
NULL value. The
function can check whether any of the function arguments can
NULL by checking the arguments'
maybe_null variable. Look at
Item_func_mod::fix_length_and_dec for a
typical example of how to do this.
All functions must be thread-safe. In other words, do not use any global or static variables in the functions without protecting them with mutexes.
If you want to return
::str(), you should set
null_value to 1 and return 0.
::str() object functions, there are
additional considerations to be aware of:
String *str argument provides a
string buffer that may be used to hold the result. (For more
information about the
String type, take a
look at the
::str() function should return the
string that holds the result, or
0 if the result is
All current string functions try to avoid allocating any memory unless absolutely necessary!