code: func (e *WindowExec) appendResult2Chunk(chk *chunk.Chunk) (err error) {
  	e.copyChk(chk)
  	remained := mathutil.Min(e.remainingRowsInChunk, e.remainingRowsInGroup)
  	e.groupRows, err = e.processor.appendResult2Chunk(e.ctx, e.groupRows, chk, remained)
  	if err != nil {
  		return err
  	}
  	e.remainingRowsInGroup -= remained
  	e.remainingRowsInChunk -= remained
  	if e.remainingRowsInGroup == 0 {
  		e.processor.resetPartialResult()
  		e.groupRows = e.groupRows[:0]
  	}
  	return nil
  }
docs: // appendResult2Chunk appends result of the window function to the result chunk.