code: func findDependedColumnNames(colDef *ast.ColumnDef) (generated bool, colsMap map[string]struct{}) {
  	colsMap = make(map[string]struct{})
  	for _, option := range colDef.Options {
  		if option.Tp == ast.ColumnOptionGenerated {
  			generated = true
  			colNames := findColumnNamesInExpr(option.Expr)
  			for _, depCol := range colNames {
  				colsMap[depCol.Name.L] = struct{}{}
  			}
  			break
  		}
  	}
  	return
  }
docs: // findDependedColumnNames returns a set of string, which indicates
  // the names of the columns that are depended by colDef.