presto-main/src/main/java/io/prestosql/operator/window/LastValueFunction.java/LastValueFunction/processRow
class LastValueFunction:    @Override
    public void processRow(BlockBuilder output, int frameStart, int frameEnd, int currentPosition)
    {
        if (frameStart < 0) {
            output.appendNull();
            return;
        }

        int valuePosition = frameEnd;

        if (ignoreNulls) {
            while (valuePosition >= frameStart) {
                if (!windowIndex.isNull(argumentChannel, valuePosition)) {
                    break;
                }

                valuePosition--;
            }

            if (valuePosition < frameStart) {
                output.appendNull();
                return;
            }
        }

        windowIndex.appendTo(argumentChannel, valuePosition, output);
    }