Function columnize_expr

datafusion/expr/src/utils.rs:782–797

Convert an expression into Column expression if it's already provided as input plan. For example, it rewrites: ```text .aggregate(vec![col("c1")], vec![sum(col("c2"))])? .project(vec![col("c1"), sum(col("c2"))? ``` Into: ```text .aggregate(vec![col("c1")], vec![sum(col("c2"))])? .project(vec![col("c1"), col("SUM(c2)")? ```

(e: Expr, input: &LogicalPlan)

/// .project(vec![col("c1"), col("SUM(c2)")?
/// ```
pub fn columnize_expr(e: Expr, input: &LogicalPlan) -> Result<Expr> {
    let output_exprs = match input.columnized_output_exprs() {
        Ok(exprs) if !exprs.is_empty() => exprs,
        _ => return Ok(e),
    };
    let exprs_map: HashMap<&Expr, Column> = output_exprs.into_iter().collect();
    e.transform_down(|node: Expr| match exprs_map.get(&node) {
        Some(column) => Ok(Transformed::new(
            Expr::Column(column.clone()),
            true,
            TreeNodeRecursion::Jump,
        )),
        None => Ok(Transformed::no(node)),
    })
    .data()
}

/// Collect all deeply nested `Expr::Column`'s. They are returned in order of
/// appearance (depth first), and may contain duplicates.