code: func (r *rangeBuilder) appendIndexRanges(origin []*IndexRange, rangePoints []rangePoint, ft *types.FieldType) []*IndexRange {
  	var newIndexRanges []*IndexRange
  	for i := 0; i < len(origin); i++ {
  		oRange := origin[i]
  		if !oRange.IsPoint(r.sc) {
  			newIndexRanges = append(newIndexRanges, oRange)
  		} else {
  			newIndexRanges = append(newIndexRanges, r.appendIndexRange(oRange, rangePoints, ft)...)
  		}
  	}
  	return newIndexRanges
  }
docs: // appendIndexRanges appends additional column ranges for multi-column index.
  // The additional column ranges can only be appended to point ranges.
  // for example we have an index (a, b), if the condition is (a > 1 and b = 2)
  // then we can not build a conjunctive ranges for this index.