presto-main/src/main/java/io/prestosql/operator/index/StreamingIndexedData.java/StreamingIndexedData/getJoinPosition
class StreamingIndexedData:    @Override
    public long getJoinPosition(int position, Page page)
    {
        checkArgument(page.getChannelCount() == indexKeyTypes.size(), "Number of blocks does not match the number of key columns");
        if (started || !matchesExpectedKey(position, page)) {
            return IndexedData.UNLOADED_INDEX_KEY;
        }
        started = true;
        if (!loadNextPage()) {
            return IndexedData.NO_MORE_POSITIONS;
        }
        return 0;
    }