public MemoryLocation getKeyAddress() {
            assert (isDefined);
            return keyMemoryLocation;
        }
Returns the address of the key defined at this position.
This points to the first byte of the key data.
Unspecified behavior if the key is not defined.
For efficiency reasons, calls to this method always returns the same MemoryLocation object.
indexr-query-opt/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java