Unparse of Joins is ignoring projections

[nuno-faria]

Describe the bug

The unparse of Join operators is ignoring the projected columns, ending up projecting everything.

Two conditions cause this to happen:

• the final projected columns match the columns in the TableScans, meaning the Projection is optimized away with the optimize_projections rule;
• the try_transform_to_simple_table_scan_with_filters function in the unparser module -- used when processing joins -- causes the TableScan projection to be discarded.

I could try to fix this by modifying the join unparser, but I would like to be sure that this is the best approach here.

To Reproduce

use datafusion::error::Result;
use datafusion::prelude::SessionContext;
use datafusion::sql::unparser::Unparser;
use datafusion::sql::unparser::dialect::PostgreSqlDialect;

#[tokio::main]
async fn main() -> Result<()> {
    let ctx = SessionContext::new();

    ctx.sql("create table test (k int, v int)")
        .await?
        .collect()
        .await?;

    let df = ctx.sql("select t1.v, t2.v from test t1, test t2").await?;

    println!("{}\n", df.logical_plan());

    let plan = df.into_optimized_plan()?;
    println!("{}\n", plan);

    println!(
        "{}",
        Unparser::new(&PostgreSqlDialect {})
            .plan_to_sql(&plan)
            .unwrap()
    );

    Ok(())
}

Projection: t1.v, t2.v
  Cross Join: 
    SubqueryAlias: t1
      TableScan: test
    SubqueryAlias: t2
      TableScan: test

Cross Join:
  SubqueryAlias: t1
    TableScan: test projection=[v]
  SubqueryAlias: t2
    TableScan: test projection=[v]

SELECT * FROM "test" AS "t1" CROSS JOIN "test" AS "t2"

Expected behavior

Output the query with the projected fields:

SELECT "t1".v, "t2".v FROM "test" AS "t1" CROSS JOIN "test" AS "t2"

Additional context

No response

[chenkovsky]

take