This section describes the different functions that you need to define when you create a simple UDF. Section 21.3.2, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.
The main xxx()
function should be declared
as shown in this section. Note that the return type and
parameters differ, depending on whether you declare the SQL
function XXX()
to return
STRING
,
INTEGER
, or
REAL
in the
CREATE FUNCTION
statement:
For STRING
functions:
char *xxx(UDF_INIT *initid, UDF_ARGS *args, char *result, unsigned long *length, char *is_null, char *error);
For INTEGER
functions:
long long xxx(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
For REAL
functions:
double xxx(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
DECIMAL
functions return string
values and should be declared the same way as
STRING
functions. ROW
functions are not implemented.
The initialization and deinitialization functions are declared like this:
my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message); void xxx_deinit(UDF_INIT *initid);
The initid
parameter is passed to all three
functions. It points to a UDF_INIT
structure that is used to communicate information between
functions. The UDF_INIT
structure members
follow. The initialization function should fill in any members
that it wishes to change. (To use the default for a member,
leave it unchanged.)
my_bool maybe_null
xxx_init()
should set
maybe_null
to 1
if
xxx()
can return
NULL
. The default value is
1
if any of the arguments are declared
maybe_null
.
unsigned int decimals
The number of decimal digits to the right of the decimal
point. The default value is the maximum number of decimal
digits in the arguments passed to the main function. For
example, if the function is passed
1.34
, 1.345
, and
1.3
, the default would be 3, because
1.345
has 3 decimal digits.
For arguments that have no fixed number of decimals, the
decimals
value is set to 31, which is 1
more than the maximum number of decimals allowed for the
DECIMAL
,
FLOAT
, and
DOUBLE
data types.
A decimals
value of 31 is used for
arguments in cases such as a
FLOAT
or
DOUBLE
column declared
without an explicit number of decimals (for example,
FLOAT
rather than
FLOAT(10,3)
) and for floating-point
constants such as 1345E-3
. It is also
used for string and other nonnumber arguments that might
be converted within the function to numeric form.
The value to which the decimals
member
is initialized is only a default. It can be changed within
the function to reflect the actual calculation performed.
The default is determined such that the largest number of
decimals of the arguments is used. If the number of
decimals is 31 for even one of the arguments, that is the
value used for decimals
.
unsigned int max_length
The maximum length of the result. The default
max_length
value differs depending on
the result type of the function. For string functions, the
default is the length of the longest argument. For integer
functions, the default is 21 digits. For real functions,
the default is 13 plus the number of decimal digits
indicated by initid->decimals
. (For
numeric functions, the length includes any sign or decimal
point characters.)
If you want to return a blob value, you can set
max_length
to 65KB or 16MB. This memory
is not allocated, but the value is used to decide which
data type to use if there is a need to temporarily store
the data.
char *ptr
A pointer that the function can use for its own purposes.
For example, functions can use
initid->ptr
to communicate allocated
memory among themselves. xxx_init()
should allocate the memory and assign it to this pointer:
initid->ptr = allocated_memory;
In xxx()
and
xxx_deinit()
, refer to
initid->ptr
to use or deallocate the
memory.
my_bool const_item
xxx_init()
should set
const_item
to 1
if
xxx()
always returns the same value and
to 0
otherwise.
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