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MySQL 5.7 Reference Manual  /  ...  /  UDF Calling Sequences for Simple Functions UDF Calling Sequences for Simple Functions

This section describes the different functions that you need to define when you create a simple UDF. Section 28.4.2, “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 permitted for the DECIMAL, FLOAT, and DOUBLE data types. This value is available as the constant NOT_FIXED_DEC in the mysql_com.h header file.

    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 NOT_FIXED_DEC 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.

User Comments
User comments in this section are, as the name implies, provided by MySQL users. The MySQL documentation team is not responsible for, nor do they endorse, any of the information provided here.
  Posted by Tim Eliseo on January 2, 2014
Despite the supposed clarification of const_item added after, and the discussion at,57613,57613, my own testing of code and reading of MySQL source reveals:

• const_item is an input/output parameter. It comes in as 1 (true) if and only if all UDF arguments are known to be constant throughout the query. That is, const_item will come into init as true only if there are no NULLs in the args array.

• If const_item is returned non-zero (true), MySQL may assume that your main function returns the same value for all calls in this context, and only call your main function once. (Or it may call your main function multiple times anyway, as I have noticed. If you care to optimize this, you should cache your return value instead of recalculating every time.) Otherwise (const_item is zero upon return), your main function will be called for every row.

This means that, in the common case where your return value is only dependent on the arguments, your init function shouldn’t touch const_item at all. You should only change const_item if:

• You might return a different value even with the same args, in which case you should set const_item to 0 (false). Random number generators or sequence number functions would be examples of this.

• The values of certain constant args will cause your function to return the same value regardless of the remaining non-constant ones, in which case you should set const_item to 1 (true). (If your return value is always the same, and independent of the value of *any* args, you should check if your function is pointless.)