code: func checkCoverIndex(idx *model.IndexInfo, ranges []*ranger.Range) bool {
  	// If the index is (c1, c2) but the query range only contains c1, it is not a unique get.
  	if !idx.Unique {
  		return false
  	}
  	for _, rg := range ranges {
  		if len(rg.LowVal) != len(idx.Columns) {
  			return false
  		}
  	}
  	return true
  }
docs: // checkCoverIndex checks whether we can pass unique info to TiKV. We should push it if and only if the length of
  // range and index are equal.