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The procedure for adding a new native function is described here. Note that you cannot add native functions to a binary distribution because the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (for example, when a new version is released), you will need to repeat the procedure with the new version.
To add a new native MySQL function, follow these steps:
sql_functions[]
array.
sql_functions[]
array and add a function that creates a function
object in `item_create.cc'. Take a look at "ABS"
and
create_funcs_abs()
for an example of this.
If the function prototype is complicated (for example takes a variable number
of arguments), you should add two lines to `sql_yacc.yy'. One
indicates the preprocessor symbol that yacc
should define (this
should be added at the beginning of the file). Then define the function
parameters and add an "item" with these parameters to the
simple_expr
parsing rule. For an example, check all occurrences
of ATAN
in `sql_yacc.yy' to see how this is done.
Item_num_func
or
Item_str_func
, depending on whether your function returns a number or a
string.
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 the above
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()
.
void Item_func_newname::fix_length_and_dec() |
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 if any of the function arguments can return
NULL
by checking the arguments maybe_null
variable. You
can take a 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, don't 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 some 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 `sql_string.h' file.)
::str()
function should return the string that holds the result or
(char*) 0
if the result is NULL
.
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