21.2.3 Adding a New Native Function

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:

  1. Add one line to lex.h that defines the function name in the sql_functions[] array.

  2. If the function prototype is simple (just takes zero, one, two, or three arguments), add a line to the sql_functions[] array in lex.h that specifies SYM(FUNC_ARGN) as the second argument (where N is the number of arguments the function takes). Also, add a function in item_create.cc that creates a function object. Look at "ABS" and create_funcs_abs() for an example of this.

    If the function prototype is not simple (for example, if it takes a variable number of arguments), you should make two changes to sql_yacc.yy. One is a line that indicates the preprocessor symbol that yacc should define; this should be added at the beginning of the file. The other is an item to be added to the simple_expr parsing rule that defines the function parameters. You will need an item for each syntax with which the function can be called. For an example that shows how this is done, check all occurrences of ATAN in sql_yacc.yy.

  3. In item_func.h, declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.

  4. 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, the Item_str_func class defines a val() function that executes atof() on the value returned by ::str().

  5. 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.

  6. You should probably also define the following object function:

    void Item_func_newname::fix_length_and_dec()
    

    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 also set maybe_null = 0 if the main function can't return a NULL value. The function can check whether any of the function arguments can return 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 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: