Access and filter in oracle plan
Based on the statistics the oprimizer has estimated this as the cheapest way to get the data. A INDEX FAST FULL SCAN reads the entire index as it is stored on disk using multiblock read. This kind of operation is prefered to other index operation   because  a high number/fraction of rows with ID1=1110 and ID2=1112 exists in the index and the data is not needed sorted. It is prefered to a full table scan because  all the data needed (ID1, ID2, MID, PARENTID) is contained in the index.