presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/startNewOutput
class UnnestBlockBuilder:    public void startNewOutput(PageBuilderStatus pageBuilderStatus, int expectedEntries)
    {
        checkState(source != null, "source is null");
        this.pageBuilderStatus = pageBuilderStatus;

        // Prepare for a new dictionary block
        this.ids = new int[expectedEntries];
        this.positionCount = 0;
        this.usingCopiedBlock = false;
    }