27.3.4.2. UDF Calling Sequences for Aggregate Functions
This section describes the different functions that you need
to define when you create an aggregate UDF.
Section 27.3.4, “Adding a New User-Defined Function”, describes the order in which
MySQL calls these functions.
-
xxx_reset()
This function is called when MySQL finds the first row in
a new group. It should reset any internal summary
variables and then use the given
UDF_ARGS argument as the first value in
your internal summary value for the group. Declare
xxx_reset() as follows:
char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
xxx_reset() is not needed or used in
MySQL 5.1, in which the UDF interface uses
xxx_clear() instead. However, you can
define both xxx_reset() and
xxx_clear() if you want to have your
UDF work with older versions of the server. (If you do
include both functions, the xxx_reset()
function in many cases can be implemented internally by
calling xxx_clear() to reset all
variables, and then calling xxx_add()
to add the UDF_ARGS argument as the
first value in the group.)
-
xxx_clear()
This function is called when MySQL needs to reset the
summary results. It is called at the beginning for each
new group but can also be called to reset the values for a
query where there were no matching rows. Declare
xxx_clear() as follows:
char *xxx_clear(UDF_INIT *initid, char *is_null, char *error);
is_null is set to point to
CHAR(0) before calling
xxx_clear().
If something went wrong, you can store a value in the
variable to which the error argument
points. error points to a single-byte
variable, not to a string buffer.
xxx_clear() is required by MySQL
5.1.
-
xxx_add()
This function is called for all rows that belong to the
same group, except for the first row. You should use it to
add the value in the UDF_ARGS argument
to your internal summary variable.
char *xxx_add(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
The xxx() function for an aggregate UDF
should be declared the same way as for a non-aggregate UDF.
See Section 27.3.4.1, “UDF Calling Sequences for Simple Functions”.
For an aggregate UDF, MySQL calls the xxx()
function after all rows in the group have been processed. You
should normally never access its UDF_ARGS
argument here but instead return a value based on your
internal summary variables.
Return value handling in xxx() should be
done the same way as for a non-aggregate UDF. See
Section 27.3.4.4, “UDF Return Values and Error Handling”.
The xxx_reset() and
xxx_add() functions handle their
UDF_ARGS argument the same way as functions
for non-aggregate UDFs. See Section 27.3.4.3, “UDF Argument Processing”.
The pointer arguments to is_null and
error are the same for all calls to
xxx_reset(),
xxx_clear(), xxx_add()
and xxx(). You can use this to remember
that you got an error or whether the xxx()
function should return NULL. You should not
store a string into *error!
error points to a single-byte variable, not
to a string buffer.
*is_null is reset for each group (before
calling xxx_clear()).
*error is never reset.
If *is_null or *error
are set when xxx() returns, MySQL returns
NULL as the result for the group function.
