This is the
This is called for each row of the table scan. When you run out of records you should return HA_ERR_END_OF_FILE. Fill buff up with the row information. The Field structure for the table is the key to getting data into buf in a manner that will allow the server to understand it.
Called from filesort.cc, records.cc, sql_handler.cc, sql_select.cc, sql_table.cc, and sql_update.cc.