Dyntex.net Support Centre: MySQL Reference Manual p.620

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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

620 MySQL Technical Reference for Version 4.0.3 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 diers 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. 9.2.2.2 UDF Calling Sequences for aggregate functions Here follows a description of the dierent functions you need to dene when you want to create an aggregate UDF function. char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error); This function is called when MySQL nds the rst row in a new group. In the function you should reset any internal summary variables and then set the given argument as the rst argument in the group. In many cases this is implemented internally by reseting all variables and then calling xxx_ add(). 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 rst 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 dene a simple UDF function. See Section 9.2.2.1 [UDF calling], page 619. 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.

MySQL Reference Manual

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