code: func (o *outerJoinEliminator) isInnerJoinKeysContainUniqueKey(innerPlan LogicalPlan, joinKeys *expression.Schema) (bool, error) {
  	for _, keyInfo := range innerPlan.Schema().Keys {
  		joinKeysContainKeyInfo := true
  		for _, col := range keyInfo {
  			columnName := &ast.ColumnName{Schema: col.DBName, Table: col.TblName, Name: col.ColName}
  			c, err := joinKeys.FindColumn(columnName)
  			if err != nil {
  				return false, err
  			}
  			if c == nil {
  				joinKeysContainKeyInfo = false
  				break
  			}
  		}
  		if joinKeysContainKeyInfo {
  			return true, nil
  		}
  	}
  	return false, nil
  }
docs: // check whether one of unique keys sets is contained by inner join keys