presto-main/src/main/java/io/prestosql/operator/unnest/Unnester.java/Unnester/startNewOutput
class Unnester:    public final void startNewOutput(PageBuilderStatus status, int expectedEntries)
    {
        for (int i = 0; i < getChannelCount(); i++) {
            unnestBlockBuilders[i].startNewOutput(status, expectedEntries);
        }
    }