26. The method of claim 25 , wherein the key of the key-value pair is constructed so that the map index collocates key-value pairs first by the path, next by the unique identifier, next by the join key, and then by the map key.
25. The method of claim 21 , further comprising storing a datum corresponding to the map into a map index as a key-value pair, wherein the value of the key-value pair is the datum, and wherein the key of the key-value pair is based on (i) a path of the datum consistent with the relational schema, (ii) a unique identifier of a first object of the retrieved objects, wherein the first object provided the datum, (iii) a join key of the map, and (iv) a map key of the datum in the map.