presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/clearCurrentOutput
class UnnestBlockBuilder:    @VisibleForTesting
    void clearCurrentOutput()
    {
        // clear dictionary block state
        ids = new int[0];
        positionCount = 0;
        usingCopiedBlock = false;

        // clear copied block state
        outputBlockBuilder = null;
        pageBuilderStatus = null;
    }