code: func (s *basePropConstSolver) tryToUpdateEQList(col *Column, con *Constant) (bool, bool) {
  	if con.Value.IsNull() {
  		return false, true
  	}
  	id := s.getColID(col)
  	oldCon := s.eqList[id]
  	if oldCon != nil {
  		return false, !oldCon.Equal(s.ctx, con)
  	}
  	s.eqList[id] = con
  	return true, false
  }
docs: // tryToUpdateEQList tries to update the eqList. When the eqList has store this column with a different constant, like
  // a = 1 and a = 2, we set the second return value to false.