Add another method to collect referenced columns from an expression

[ygf11]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently we use expr_to_columns to collect referenced columns from an expression. The usage is like:

  let mut using_columns = HashSet::new();
  expr_to_columns(expr, &mut using_columns)?;

It is not convenient to use, because we need create a new HashSet everytime we invoke it. And the most usages of it is to collect from an single expression.

Describe the solution you'd like
Add a new method to Expr for collecting columns, like:

   pub fn to_columns(&self) -> Result<HashSet<Column>> {
        let mut using_columns = HashSet::new();
        expr_to_columns(self, &mut using_columns)?;

        Ok(using_columns)
    }

The usage will be like:

  let using_columns = expr.to_columns()?;
  ...

Describe alternatives you've considered

Additional context