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9 Extending MySQL

9.1 Adding New Functions to MySQL

There are two ways to add new functions to MySQL:

Each method has advantages and disadvantages:

Whichever method you use to add new functions, they may be used just like native functions such as ABS() or SOUNDEX().

9.1.1 CREATE FUNCTION/DROP FUNCTION Syntax

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

DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new function that works like native (built in) MySQL functions such as ABS() and CONCAT().

AGGREGATE is a new option for MySQL Version 3.23. An AGGREGATE function works exactly like a native MySQL GROUP function like SUM or COUNT().

CREATE FUNCTION saves the function's name, type, and shared library name in the mysql.func system table. You must have the insert and delete privileges for the mysql database to create and drop functions.

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. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

For instructions on writing user-definable functions, see section 9.1 Adding New Functions to MySQL. For the UDF mechanism to work, functions must be written in C or C++, your operating system must support dynamic loading and you must have compiled mysqld dynamically (not statically).

9.1.2 Adding a New User-definable Function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file `sql/udf_example.cc' that defines 5 new functions. Consult this file to see how UDF calling conventions work.

For mysqld to be able to use UDF functions, you should configure MySQL with --with-mysqld-ldflags=-rdynamic The reason is that to on many platforms (including Linux) you can load a dynamic library (with dlopen()) from a static linked program, which you would get if you are using --with-mysqld-ldflags=-all-static If you want to use an UDF that needs to access symbols from mysqld (like the methaphone example in `sql/udf_example.cc' that uses default_charset_info), you must link the program with -rdynamic. (see man dlopen).

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the discussion below, the name ``xxx'' is used for an example function name. To distinquish between SQL and C/C++ usage, XXX() (uppercase) indicates a SQL function call, and xxx() (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the interface for XXX() are:

xxx() (required)
The main function. This is where the function result is computed. The correspondence between the SQL type and return type of your C/C++ function is shown below:
SQL type C/C++ type
STRING char *
INTEGER long long
REAL double
xxx_init() (optional)
The initialization function for xxx(). It can be used to:
xxx_deinit() (optional)
The deinitialization function for xxx(). It should deallocate any memory allocated by the initialization function.

When a SQL statement invokes XXX(), MySQL calls the initialization function xxx_init() to let it perform any required setup, such as argument checking or memory allocation. If xxx_init() returns an error, the SQL statement is aborted with an error message and the main and deinitialization functions are not called. Otherwise, the main function xxx() is called once for each row. After all rows have been processed, the deinitialization function xxx_deinit() is called so it can perform any required cleanup.

All functions must be thread safe (not just the main function, but the initialization and deinitialization functions as well). This means that you are not allowed to allocate any global or static variables that change! If you need memory, you should allocate it in xxx_init() and free it in xxx_deinit().

9.1.2.1 UDF Calling Sequences

The main function should be declared as shown below. Note that the return type and parameters differ, depending on whether you will 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 are listed below. 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
Number of decimals. The default value is the maximum number of decimals 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 decimals.
unsigned int max_length
The maximum length of the string result. The default 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 decimals indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.)
char *ptr
A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory between functions. In xxx_init(), 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.

9.1.2.2 Argument Processing

The args parameter points to a UDF_ARGS structure that thas the members listed below:

unsigned int arg_count
The number of arguments. Check this value in the initialization function if you want your function to be called with a particular number of arguments. For example: 
if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}
enum Item_result *arg_type
The types for each argument. The possible type values are STRING_RESULT, INT_RESULT, and REAL_RESULT. To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example: 
if (args->arg_type[0] != STRING_RESULT ||
    args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}
As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify coercion of the first two arguments to string and integer, do this in xxx_init(): 
args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;
char **args
args->args communicates information to the initialization function about the general nature of the arguments your function was called with. For a constant argument i, args->args[i] points to the argument value. (See below for instructions on how to access the value properly.) For a non-constant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A non-constant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with non-constant arguments. For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed. Functions can refer to an argument i as follows:
unsigned long *lengths
For the initialization function, the lengths array indicates the maximum string length for each argument. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

9.1.2.3 Return Values and Error Handling

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 will be 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 functions should return a pointer to the result and store the length of the string in the length arguments. result is a buffer at least 255 bytes long. Set these to the contents and length of the return value. For example: 

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

If your string functions that 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 9.1.2.1 UDF Calling Sequences.

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 the error parameter 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() will not even be called for subsequent rows.) NOTE: In MySQL versions prior to 3.22.10, you should set both *error and *is_null: 

*error = 1;
*is_null = 1;

9.1.2.4 Compiling and Installing User-definable Functions

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 `udf_example.cc' that is included in the MySQL source distribution. This 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 myfunc.cc

You can easily find out 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.)

Once 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 searched by ld, such as `/usr/lib'. On many systems, you can set the LD_LIBRARY or LD_LIBRARY_PATH environment variable to point at the directory where you have your UDF function files. The dlopen manual page tells you which variable you should use on your system. You should set this in mysql.server or safe_mysqld and restart mysqld.

After the library is installed, notify mysqld about the new functions with these commands: 

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";

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;

The CREATE FUNCTION and DROP FUNCTION statements update the system table func 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 already 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 will continue to use the old version.

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. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

9.1.3 Adding a New Native Function

The procedure for adding a new native function is described below. Note that you cannot add native functions to a binary distribution because the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (for example, when a new version is released), you will need to repeat the procedure with the new version.

To add a new native MySQL function, follow these steps:

  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_ARG#) (where # 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 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 the above 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 if 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.

All functions must be thread safe (In other words, don't use any global or static variables in the functions without protecting them with mutexes).

If you want to return NULL, from ::val(), ::val_int() or ::str() you should set null_value to 1 and return 0.

For ::str() object functions, there are some additional considerations to be aware of:

9.2 Adding New Procedures to MySQL

In MySQL, you can define a procedure in C++ that can access and modify the data in a query before it is sent to the client. The modification can be done on row-by-row or GROUP BY level.

We have created an example procedure in MySQL Version 3.23 to show you what can be done.

Additionally we recommend you to take a look at 'mylua', which you can find in the Contrib directory. See section D Contributed Programs. Which this you can use the LUA language to load a procedure at runtime into mysqld.

9.2.1 Procedure Analyse

analyse([max elements,[max memory]])

This procedure is defined in the `sql/sql_analyse.cc'. This examines the result from your query and returns an analysis of the results: 

SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])

9.2.2 Writing a Procedure

For the moment, the only documentation for this is the source.

You can find all information about procedures by examining the following files:

9.3 MySQL Internals

This chapter describes a lot of things that you need to know when working on the MySQL code. If you plan to contribute to MySQL development, want to have access to the bleeding-edge in-between versions code, or just want to keep track of development, follow the instructions in See section 2.3.4 Installing from the Development Source Tree. If you are interested in MySQL internals, you should also subscribe to internals@lists.mysql.com. This is a relatively low traffic list, in comparison with mysql@lists.mysql.com.

9.3.1 MySQL Threads

The MySQL server creates the following threads:

mysqladmin processlist only shows the connection, INSERT DELAYED, and replication threads.

9.3.2 MySQL Test Suite

Until recently, our main full-coverage test suite was based on proprietary customer data and for that reason has not been publicly available. The only publicly available part of our testing process consisted of the crash-me test, a Perl DBI/DBD benchmark found in the sql-bench directory, and miscellaneous tests located in tests directory. The lack of a standardized publicly available test suite has made it difficult for our users, as well developers, to do regression tests on the MySQL code. To address this problem, we have created a new test system that is included in the source and binary distributions starting in Version 3.23.29.

The current set of test cases doesn't test everything in MySQL, but it should catch most obvious bugs in the SQL processing code, OS/library issues, and is quite thorough in testing replication. Our eventual goal is to have the tests cover 100% of the code. We welcome contributions to our test suite. You may especially want to contribute tests that examine the functionality critical to your system, as this will ensure that all future MySQL releases will work well with your applications.

9.3.2.1 Running the MySQL Test Suite

The test system consist of a test language interpreter (mysqltest), a shell script to run all tests(mysql-test-run), the actual test cases written in a special test language, and their expected results. To run the test suite on your system after a build, type make test or mysql-test/mysql-test-run from the source root. If you have installed a binary distribution, cd to the install root (eg. /usr/local/mysql), and do scripts/mysql-test-run. All tests should succeed. If not, you should try to find out why and report the problem if this is a bug in MySQL. See section 9.3.2.3 Reporting Bugs in the MySQL Test Suite.

If you have a copy of mysqld running on the machine where you want to run the test suite you do not have to stop it, as long as it is not using ports 9306 and 9307. If one of those ports is taken, you should edit mysql-test-run and change the values of the master and/or slave port to one that is available.

You can run one individual test case with mysql-test/mysql-test-run test_name.

If one test fails, you should test running mysql-test-run with the --force option to check if any other tests fails.

9.3.2.2 Extending the MySQL Test Suite

You can use the mysqltest language to write your own test cases. Unfortunately, we have not yet written full documentation for it - we plan to do this shortly. You can, however, look at our current test cases and use them as an example. The following points should help you get started:

9.3.2.3 Reporting Bugs in the MySQL Test Suite

If your MySQL version doesn't pass the test suite you should do the following:

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