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:
Create a subclass for the function in
item_create.cc:If the function takes a fixed number of arguments, create a subclass of
Create_func_arg0,Create_func_arg1,Create_func_arg2, orCreate_func_arg3, respectively, depending on whether the function takes zero, one, two, or three arguments. For examples, see theCreate_func_uuid,Create_func_abs,Create_func_pow, andCreate_func_lpadclasses.If the function takes a variable number of arguments, create a subclass of
Create_native_func. For an example, seeCreate_func_concat.
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
"LCASE"and"LOWER", which are aliases forCreate_func_lcase.In
item_func.h, declare a class inheriting fromItem_num_funcorItem_str_func, depending on whether your function returns a number or a string.In
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, theItem_str_funcclass defines aval()function that executesatof()on the value returned by::str().If the function is nondeterministic, include the following statement in the item constructor to indicate that function results should not be cached:
current_thd->lex->safe_to_cache_query=0;
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:
void Item_func_newname::fix_length_and_dec()
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 setmaybe_null = 0if the main function can't return aNULLvalue. The function can check whether any of the function arguments can returnNULLby checking the arguments'maybe_nullvariable. Look atItem_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 NULL from
::val(), ::val_int(), or
::str(), you should set
null_value to 1 and return 0.
For ::str() object functions, there are
additional considerations to be aware of:
The
String *strargument provides a string buffer that may be used to hold the result. (For more information about theStringtype, take a look at thesql_string.hfile.)The
::str()function should return the string that holds the result, or(char*) 0if the result isNULL.All current string functions try to avoid allocating any memory unless absolutely necessary!