@Override
    public RecordPointerAndKeyPrefix getKey(LongArray data, int pos) {
        // Since we re-use keys, this method shouldn't be called.
        throw new UnsupportedOperationException();
    }
Supports sorting an array of (record pointer, key prefix) pairs.
Used in {@link UnsafeInMemorySorter}.

Within each long[] buffer, position {@code 2 * i} holds a pointer pointer to the record at
index {@code i}, while position {@code 2 * i + 1} in the array holds an 8-byte key prefix.
indexr-query-opt/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeSortDataFormat.java