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

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.

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.

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().

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_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.