code: func colMatch(a *ast.ColumnName, b *ast.ColumnName) bool {
  	if a.Schema.L == "" || a.Schema.L == b.Schema.L {
  		if a.Table.L == "" || a.Table.L == b.Table.L {
  			return a.Name.L == b.Name.L
  		}
  	}
  	return false
  }
docs: // colMatch means that if a match b, e.g. t.a can match test.t.a but test.t.a can't match t.a.
  // Because column a want column from database test exactly.