Explicitly allocated descriptors can be freed either explicitly, by calling SQLFreeHandle with HandleType of SQL_HANDLE_DESC, or implicitly, when the connection handle is freed. When an explicitly allocated descriptor is freed, all statement handles to which the freed descriptor applied automatically revert to the descriptors implicitly allocated for them.
Implicitly allocated descriptors can be freed only by calling SQLDisconnect, which drops any statements or descriptors open on the connection, or by calling SQLFreeHandle with a HandleType of SQL_HANDLE_STMT to free a statement handle and all the implicitly allocated descriptors associated with the statement. An implicitly allocated descriptor cannot be freed by calling SQLFreeHandle with a HandleType of SQL_HANDLE_DESC.
Even when freed, an implicitly allocated descriptor remains valid, and SQLGetDescField can be called on its fields.