To add a 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 native function will be referred to in statements that will be replicated to replicas, you must ensure that every replica also has the function available. Otherwise, replication will fail on the replicas when they attempt to invoke the function.
To add a native function, follow these steps to modify source
files in the
Create a subclass for the function in
If the function takes a fixed number of arguments, create a subclass of
Create_func_arg3, respectively, 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, see
To provide a name by which the function can be referred to in SQL statements, register the name in
item_create.ccby adding a line to this array:
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 inheriting from
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 (like
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_funcclass 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_lengthbased on the given arguments.
max_lengthis the maximum number of characters the function may return. This function should also set
maybe_null = 0if the main function cannot return a
NULLvalue. The function can check whether any of the function arguments can return
NULLby checking the arguments'
maybe_nullvariable. Look at
Item_func_mod::fix_length_and_decfor 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, these
additional considerations apply:
String *strargument provides a string buffer that may be used to hold the result. (For more information about the
Stringtype, take a look at the
::str()function should return the string that holds the result, or
(char*) 0if the result is
All current string functions try to avoid allocating any memory unless absolutely necessary!