Dyntex.net Support Centre: MySQL Reference Manual p.619

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

Chapter 9: Extending MySQL 619 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 rst 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(). 9.2.2.1 UDF Calling Sequences for simple functions The main function should be declared as shown here. Note that the return type and parameters dier, 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 ll 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.

MySQL Reference Manual

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