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29.4.2.2 UDF Calling Sequences for Aggregate Functions
This section describes the different functions that you need to define when you create an aggregate UDF. Section 29.4.2, “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. Declarexxx_reset()
as follows:void xxx_reset(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
xxx_reset()
is not needed or used in MySQL 8.0, in which the UDF interface usesxxx_clear()
instead. However, you can define bothxxx_reset()
andxxx_clear()
if you want to have your UDF work with older versions of the server. (If you do include both functions, thexxx_reset()
function in many cases can be implemented internally by callingxxx_clear()
to reset all variables, and then callingxxx_add()
to add theUDF_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:void xxx_clear(UDF_INIT *initid, char *is_null, char *error);
is_null
is set to point toCHAR(0)
before callingxxx_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 8.0.xxx_add()
This function is called for all rows that belong to the same group. You should use it to add the value in the
UDF_ARGS
argument to your internal summary variable.void 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 nonaggregate UDF. See
Section 29.4.2.1, “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 nonaggregate UDF. See
Section 29.4.2.4, “UDF Return Values and Error Handling”.
The xxx_reset()
and
xxx_add()
functions handle their
UDF_ARGS
argument the same way as functions
for nonaggregate UDFs. See Section 29.4.2.3, “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.
Document created the 26/06/2006, last modified the 26/10/2018
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