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12. Extending MySQL

12.1 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 Installing from the Development Source Tree. If you are interested in MySQL internals, you should also subscribe to our internals mailing list. This list is relatively low traffic. For details on how to subscribe, please see The MySQL Mailing Lists. All developers at MySQL AB are on the internals list and we help other people who are working on the MySQL code. Feel free to use this list both to ask questions about the code and to send patches that you would like to contribute to the MySQL project!

12.1.1 MySQL Threads

The MySQL server creates the following threads:

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

12.1.2 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 standardised 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. 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 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. 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. You can, however, look at our current test cases and use them as an example. The following points should help you get started: Reporting Bugs in the MySQL Test Suite

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

12.2 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().


       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 function 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 initialisation 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 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).

Note that to make AGGREGATE work, you must have a mysql.func table that contains the column type. If this is not the case, you should run the script mysql_fix_privilege_tables to get this fixed.

12.2.2 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 metaphone example in `sql/udf_example.cc' that uses default_charset_info), you must link the program with -rdynamic (see man dlopen).

If you are using a precompiled version of the server, use MySQL-Max, which supports dynamic loading.

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 distinguish between SQL and C/C++ usage, XXX() (uppercase) indicates an 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 here:

SQL type

C/C++ type


char *


long long



xxx_init() (optional)

The initialisation function for xxx(). It can be used to:

xxx_deinit() (optional)

The deinitialisation function for xxx(). It should deallocate any memory allocated by the initialisation function.

When an SQL statement invokes XXX(), MySQL calls the initialisation 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 deinitialisation functions are not called. Otherwise, the main function xxx() is called once for each row. After all rows have been processed, the deinitialisation function xxx_deinit() is called so it can perform any required cleanup.

For aggregate functions (like SUM()), you must also provide the following functions:

xxx_reset() (required)

Reset sum and insert the argument as the initial value for a new group.

xxx_add() (required)

Add the argument to the old sum.

When using aggregate UDFs, MySQL works the following way:

  1. Call xxx_init() to let the aggregate function allocate the memory it will need to store results.
  2. Sort the table according to the GROUP BY expression.
  3. For the first row in a new group, call the xxx_reset() function.
  4. For each new row that belongs in the same group, call the xxx_add() function.
  5. When the group changes or after the last row has been processed, call xxx() to get the result for the aggregate.
  6. Repeat 3-5 until all rows has been processed
  7. Call xxx_deinit() to let the UDF free any memory it has allocated.

All functions must be thread-safe (not just the main function, but the initialisation and deinitialisation 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(). UDF Calling Sequences for simple functions

The main function should be declared as shown here. 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 initialisation and deinitialisation 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 initialisation 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.)

If you want to return a blob, you can set this to 65K or 16M; this memory is not allocated but used to decide which column type to use if there is a need to temporary 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 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. UDF Calling Sequences for aggregate functions

Here follows a description of the different functions you need to define when you want to create an aggregate UDF function.

Note that the following function is NOT needed or used by MySQL 4.1.1. You can keep still have define his function if you want to have your code work with both MySQL 4.0 and MySQL 4.1.1

char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
                char *is_null, char *error);

This function is called when MySQL finds the first row in a new group. In the function you should reset any internal summary variables and then set the given argument as the first argument in the group.

In many cases this is implemented internally by reseting all variables (for example by calling xxx_clear() and then calling xxx_add().

The following function is only required by MySQL 4.1.1 and above:

char *xxx_clear(UDF_INIT *initid, char *is_null, char *error);

This function is called when MySQL needs to reset the summary results. This will be called at the beginning for each new group but can also be called to reset the values for a query where there was no matching rows. is_null will be set to point to CHAR(0) before calling xxx_clear().

You can use the error pointer to store a byte if something went wrong .

char *xxx_add(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

This function is called for all rows that belongs to the same group, except for the first row. In this you should add the value in UDF_ARGS to your internal summary variable.

The xxx() function should be declared identical as when you define a simple UDF function. See section UDF Calling Sequences for simple functions.

This function is called when all rows in the group has been processed. You should normally never access the args variable here but return your value based on your internal summary variables.

All argument processing in xxx_reset() and xxx_add() should be done identically as for normal UDFs. See section Argument Processing.

The return value handling in xxx() should be done identically as for a normal UDF. See section Return Values and Error Handling.

The pointer argument to is_null and error is 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 if the xxx() function should return NULL. Note that you should not store a string into *error! This is just a 1 byte flag!

is_null is reset for each group (before calling xxx_clear() error is never reset.

If isnull or error are set after xxx() then MySQL will return NULL as the result for the group function. Argument Processing

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

unsigned int arg_count

The number of arguments. Check this value in the initialisation 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 initialisation 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 initialisation 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 initialisation 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 initialisation function, the lengths array indicates the maximum string length for each argument. You should not change these. 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 initialisation function). Return Values and Error Handling

The initialisation 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.

Set these 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 calc function is 255 byte big. 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 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 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; 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 the dynamic linker ld, such as `/usr/lib' or add the directory in which you placed the shared object to the linker configuration file (for example, `/etc/ld.so.conf').

On many systems, you can also 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 mysqld_safe startup scripts 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";
    ->        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 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 initialisation is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

12.2.3 Adding a New Native Function

The procedure for adding a new native function is described here. 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:

12.3 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. With this you can use the LUA language to load a procedure at runtime into mysqld.

12.3.1 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]])

12.3.2 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:

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