24.2. Adding New Functions to MySQL

• Functions can return string, integer, or real values.

• You can define simple functions that operate on a single row at a time, or aggregate functions that operate on groups of rows.

• Information is provided to functions that enables them to check the number and types of the arguments passed to them.

• You can tell MySQL to coerce arguments to a given type before passing them to a function.

• You can indicate that a function returns NULL or that an error occurred.

CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|INTEGER|REAL|DECIMAL}
    SONAME shared_library_name

DROP FUNCTION function_name

• xxx() (required)

  The main function. This is where the function result is computed. The correspondence between the SQL function data type and the return type of your C/C++ function is shown here:

  SQL Type	C/C++ Type
  STRING	char *
  INTEGER	long long
  REAL	double

  It is also possible to declare a DECIMAL function, but currently the value is returned as a string, so you should write the UDF as though it were a STRING function.

• xxx_init() (optional)

  The initialization function for xxx(). It can be used for the following purposes:

  • To check the number of arguments to XXX().

  • To check that the arguments are of a required type or, alternatively, to tell MySQL to coerce arguments to the types you want when the main function is called.

  • To allocate any memory required by the main function.

  • To specify the maximum length of the result.

  • To specify (for REAL functions) the maximum number of decimal places in the result.

  • To specify whether the result can be NULL.

• xxx_deinit() (optional)

  The deinitialization function for xxx(). It should deallocate any memory allocated by the initialization function.

• xxx_clear() (required in 5.0)

  Reset the current aggregate value but do not insert the argument as the initial aggregate value for a new group.

• xxx_add() (required)

  Add the argument to the current aggregate value.

1. Call xxx_init() to let the aggregate function allocate any memory it needs for storing results.

2. Sort the table according to the GROUP BY expression.

3. Call xxx_clear() for the first row in each new group.

4. Call xxx_add() for each new row that belongs in the same group.

5. Call xxx() to get the result for the aggregate when the group changes or after the last row has been processed.

6. Repeat 3-5 until all rows has been processed

7. Call xxx_deinit() to let the UDF free any memory it has allocated.

This section describes the different functions that you need to define when you create a simple UDF. Section 24.2.4, “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);

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.)

This section describes the different functions that you need to define when you create an aggregate UDF. Section 24.2.4, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.

The xxx() function for an aggregate UDF should be declared the same way as for a non-aggregate UDF. See Section 24.2.4.1, “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 24.2.4.4, “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 24.2.4.3, “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.

The args parameter points to a UDF_ARGS structure that has the members listed here:

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message is returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. A string function should return a pointer to the result and set *result and *length to the contents and length of the return value. For example:

memcpy(result, "result string", 13);
*length = 13;

The result buffer that is passed to the xxx() function is 255 bytes long. If your result fits in this, you don't have to worry about memory allocation for results.

If your string function needs to return a string longer than 255 bytes, you must allocate the space for it with malloc() in your xxx_init() function or your xxx() function and free it in your xxx_deinit() function. You can store the allocated memory in the ptr slot in the UDF_INIT structure for reuse by future xxx() calls. See Section 24.2.4.1, “UDF Calling Sequences for Simple Functions”.

To indicate a return value of NULL in the main function, set *is_null to 1:

*is_null = 1;

To indicate an error return in the main function, set *error to 1:

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() is not even called for subsequent rows.)

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file sql/udf_example.cc that is included in the MySQL source distribution.

The immediately following instructions are for Unix. Instructions for Windows are given later in this section.

The udf_example.cc file contains the following functions:

A dynamically loadable file should be compiled as a sharable object file, using a command something like this:

shell> gcc -shared -o udf_example.so udf_example.cc

If you are using gcc, you should be able to create udf_example.so with a simpler command:

shell> make udf_example.so

You can easily determine the correct compiler options for your system by running this command in the sql directory of your MySQL source tree:

shell> make udf_example.o

You should run a compile command similar to the one that make displays, except that you should remove the -c option near the end of the line and add -o udf_example.so to the end of the line. (On some systems, you may need to leave the -c on the command.)

After you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from udf_example.cc produces a file named something like udf_example.so (the exact name may vary from platform to platform). Copy this file to some directory such as /usr/lib that searched by your system's dynamic (runtime) linker, or add the directory in which you placed the shared object to the linker configuration file (for example, /etc/ld.so.conf).

The dynamic linker name is system-specific (for example, ld-elf.so.1 on FreeBSD, ld.so on Linux, or dyld on Mac OS X). Consult your system documentation for information about the linker name and how to configure it.

On many systems, you can also set the LD_LIBRARY or LD_LIBRARY_PATH environment variable to point at the directory where you have the files for your UDF. The dlopen manual page tells you which variable you should use on your system. You should set this in mysql.server or mysqld_safe startup scripts and restart mysqld.

On some systems, the ldconfig program that configures the dynamic linker does not recognize a shared object unless its name begins with lib. In this case you should rename a file such as udf_example.so to libudf_example.so.

On Windows, you can compile user-defined functions by using the following procedure:

After the shared object file has been installed, notify mysqld about the new functions with these statements:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION reverse_lookup
    ->        RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE AGGREGATE FUNCTION avgcost
    ->        RETURNS REAL SONAME 'udf_example.so';

Functions can be deleted using DROP FUNCTION:

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;
mysql> DROP FUNCTION avgcost;

The CREATE FUNCTION and DROP FUNCTION statements update the func system table in the mysql database. The function's name, type and shared library name are saved in the table. You must have the INSERT and DELETE privileges for the mysql database to create and drop functions.

You should not use CREATE FUNCTION to add a function that has previously been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise, the server continues to use the old version.

An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION. All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable.

MySQL takes the following measures to prevent misuse of user-defined functions.

You must have the INSERT privilege to be able to use CREATE FUNCTION and the DELETE privilege to be able to use DROP FUNCTION. This is necessary because these statements add and delete rows from the mysql.func table.

UDFs should have at least one symbol defined in addition to the xxx symbol that corresponds to the main xxx() function. These auxiliary symbols correspond to the xxx_init(), xxx_deinit(), xxx_reset(), xxx_clear(), and xxx_add() functions. As of MySQL 5.0.3, mysqld supports an --allow-suspicious-udfs option that controls whether UDFs that have only an xxx symbol can be loaded. By default, the option is off, to prevent attempts at loading functions from shared object files other than those containing legitimate UDFs. If you have older UDFs that contain only the xxx symbol and that cannot be recompiled to include an auxiliary symbol, it may be necessary to specify the --allow-suspicious-udfs option. Otherwise, you should avoid enabling this capability.

UDF object files cannot be placed in arbitrary directories. They must be located in some system directory that the dynamic linker is configured to search. To enforce this restriction and prevent attempts at specifying pathnames outside of directories searched by the dynamic linker, MySQL checks the shared object file name specified in CREATE FUNCTION statements for pathname delimiter characters. As of MySQL 5.0.3, MySQL also checks for pathname delimiters in filenames stored in the mysql.func table when it loads functions. This prevents attempts at specifying illegitimate pathnames through direct manipulation of the mysql.func table. For information about UDFs and the runtime linker, see Section 24.2.4.5, “Compiling and Installing User-Defined Functions”.

1. Add one line to lex.h that defines the function name in the sql_functions[] array.

2. If the function prototype is simple (just takes zero, one, two or three arguments), you should in lex.h specify SYM(FUNC_ARGN) (where N is the number of arguments) as the second argument in the 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, if it 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.

3. In item_func.h, declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.

4. In item_func.cc, add one of the following declarations, depending on whether you are defining a numeric or string function:

   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 these 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().

5. You should probably also define the following object function:

   void Item_func_newname::fix_length_and_dec()
   

   This function should at least calculate 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 whether 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.