code: func (c *Chunk) LowerBound(colIdx int, d *types.Datum) (index int, match bool) {
  	index = sort.Search(c.NumRows(), func(i int) bool {
  		cmp := Compare(c.GetRow(i), colIdx, d)
  		if cmp == 0 {
  			match = true
  		}
  		return cmp >= 0
  	})
  	return
  }
docs: // LowerBound searches on the non-decreasing column colIdx,
  // returns the smallest index i such that the value at row i is not less than `d`.