MySQL Internals Manual  /  ...  /  change_columns()

18.1.2 change_columns()

Prototype: virtual bool change_columns(List<Item> &field_list);

Here you can change the structure of the result field list, for example, you can add fields to the field_list or replace the queries result fields by something completely different alltogether (like PROCEDURE ANALYSE() does).

An example that adds an INTEGER field at the end of the field list:

bool proc_rownum::change_columns(List<Item> &field_list)
 {
   DBUG_ENTER("proc_rownum::change_columns");

   // create a new column item
   row_num_column = new Item_proc_int("RowNum");

   // and attach it to the column list
   field_list.push_back(row_num_column);
 
   DBUG_RETURN(0);
 }

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