presto-main/src/main/java/io/prestosql/operator/index/IndexSnapshot.java/IndexSnapshot/getJoinPosition
class IndexSnapshot:    @Override
    public long getJoinPosition(int position, Page page)
    {
        long joinPosition = values.getJoinPosition(position, page, page);
        if (joinPosition < 0) {
            if (missingKeys.getJoinPosition(position, page, page) < 0) {
                return UNLOADED_INDEX_KEY;
            }
            return NO_MORE_POSITIONS;
        }
        return joinPosition;
    }