[jira] [Created] (ARROW-9520) [Rust] [DataFusion] Can't alias an aggregate expression

["Jorge (Jira)" <ji...@apache.org>]

Jorge created ARROW-9520:
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             Summary: [Rust] [DataFusion] Can't alias an aggregate expression
                 Key: ARROW-9520
                 URL: https://issues.apache.org/jira/browse/ARROW-9520
             Project: Apache Arrow
          Issue Type: Bug
          Components: Rust - DataFusion
            Reporter: Jorge


The following test (on execute) fails:

{code}
    #[test]
    fn aggregate_with_alias() -> Result<()> {
        let results = execute("SELECT c1, COUNT(c2) AS count FROM test GROUP BY c1", 4)?;

        assert_eq!(field_names(batch), vec!["c1", "count"]);
        let expected = vec!["0,10", "1,10", "2,10", "3,10"];
        let mut rows = test::format_batch(&batch);
        rows.sort();
        assert_eq!(rows, expected);
        Ok(())
    }
{code}

The root cause is that, in {{sql::planner}}, we interpret {{COUNT(c2) AS count}} as An {{Expr::Alias}}, which fails the {{is_aggregate_expr}} condition, thus being interpreted as grouped expression instead of an aggregated expression. This raises the Error

{{General("Projection references non-aggregate values")}}

The planner could interpret the statement above as two steps: an aggregation followed by a projection. Alternatively, we can allow aliases to be valid aggregation expressions.



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