func (expr *ColumnItem) Walk(_ Visitor) Expr {
	// TODO(knz): When ARRAY is supported, this must be extended
	// to recurse into the index expressions of the ColumnItems' Selector.
	return expr
}
// Walk implements the Expr interface.
Walk implements the Expr interface.
// Walk implements the Expr interface .
func (expr *ColumnItem) Walk(_ Visitor) Expr {
	return expr
}
func ( expr * ColumnItem ) Walk ( _ Visitor ) Expr { return expr }
Walk implements the Expr interface.
Walk implements the Expr interface .
// TODO(knz): When ARRAY is supported, this must be extended // to recurse into the index expressions of the ColumnItems' Selector.