presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/startUsingCopiedBlock
class UnnestBlockBuilder:    private void startUsingCopiedBlock()
    {
        requireNonNull(pageBuilderStatus, "pageBuilderStatus is null");
        // initialize state for copied block
        outputBlockBuilder = type.createBlockBuilder(pageBuilderStatus.createBlockBuilderStatus(), ids.length);

        // Transfer elements from ids[] to output page builder
        for (int i = 0; i < positionCount; i++) {
            type.appendTo(source, ids[i], outputBlockBuilder);
        }

        // reset state for dictionary block
        ids = new int[0];
        positionCount = 0;

        usingCopiedBlock = true;
    }