Creating User-defined Functions

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User-defined functions allow MariaDB to be extended with a new function that works like a native (built-in) MariaDB function such as ABS() or CONCAT(). There are alternative ways to add a new function: writing a native function (which requires modifying and compiling the server source code), or writing a stored function.

Statements making use of user-defined functions are not safe for replication.

Functions are written in C or C++, and to make use of them, the operating system must support dynamic loading.

Each new SQL function requires corresponding functions written in C/C++. In the list below, at least the main function - x() - and one other, are required. x should be replaced by the name of the function you are creating.

Simple functions

x()

Required for all UDF's, this is where the results are calculated.

C/C++ typeSQL type
char *STRING
long longINTEGER
doubleREAL

DECIMAL functions return string values, and so should be written accordingly. It is not possible to create ROW functions.

x_init()

Initialization function for x(). Can be used for the following:

  • Check the number of arguments to X() (the SQL equivalent).
  • Verify the argument types, or to force arguments to be of a particular type after the function is called.
  • Specify whether the result can be NULL.
  • Specify the maximum result length.
  • For REAL functions, specify the maximum number of decimals for the result.
  • Allocate any required memory. To verify that the arguments are of a required type or, alternatively, to tell MySQL to coerce arguments to the required types when the main function is called.

x_deinit()

De-initialization function for x(). Used to de-allocate memory that was allocated in x_init().

Description

Each time the SQL function X() is called:

  • MariaDB will first call the C/C++ initialization function, x_init(), assuming it exists. All setup will be performed, and if it returns an error, the SQL statement is aborted and no further functions are called.
  • If there is no x_init() function, or it has been called and did not return an error, x() is then called once per row.
  • After all rows have finished processing, x_deinit() is called, if present, to clean up by de-allocating any memory that was allocated in x_init().

Aggregate functions

The following functions are required for aggregate functions, such as AVG() and SUM().

x_clear()

Used to reset the current aggregate, but without inserting the argument as the initial aggregate value for the new group.

x_add()

Used to add the argument to the current aggregate.

Description

Each time the aggregate SQL function X() is called:

  • MariaDB will first call the C/C++ initialization function, x_init(), assuming it exists. All setup will be performed, and if it returns an error, the SQL statement is aborted and no further functions are called.
  • If there is no x_init() function, or it has been called and did not return an error, x() is then called once per row.
  • After all rows have finished processing, x_deinit() is called, if present, to clean up by de-allocating any memory that was allocated in x_init().
  • MariaDB will first call the C/C++ initialization function, x_init(), assuming it exists. All setup will be performed, and if it returns an error, the SQL statement is aborted and no further functions are called.
  • The table is sorted according to the GROUP BY expression.
  • x_clear() is called for the first row of each new group.
  • x_add() is called once per row for each row in the same group.
  • x() is called when the group changes, or after the last row, to get the aggregate result.
  • The latter three steps are repeated until all rows have been processed.
  • After all rows have finished processing, x_deinit() is called, if present, to clean up by de-allocating any memory that was allocated in x_init().

Examples

For an example, see sql/udf_example.cc in the source tree. For a collection of existing UDFs see http://www.mysqludf.org/.

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