code: func isDefaultValNowSymFunc(expr ast.ExprNode) bool {
  	if funcCall, ok := expr.(*ast.FuncCallExpr); ok {
  		// Default value NOW() is transformed to CURRENT_TIMESTAMP() in parser.
  		if funcCall.FnName.L == ast.CurrentTimestamp {
  			return true
  		}
  	}
  	return false
  }
docs: // isDefaultValNowSymFunc checks whether defaul value is a NOW() builtin function.