GroupingSet Expr --> String Support

[alamb]

Is your feature request related to a problem or challenge?

Part of #9726 to complete the Expr

Converting Expr back to SQL is valuable for several usecases such as using DataFusion in federated databases (e.g. generate SQL to push down) or using DataFusion's expr APIs to programatically create SQL.

Describe the solution you'd like

Support converting SQL syntax like this:

TODO I AM NOT SURE HOW TO MAKE THIS IN SQL

Describe alternatives you've considered

The basic pattern is:

1. Implement the Expr --> AST reverse code in Unparser::expr_to_sql(source link)
2. Add a test to expr_to_sql_ok

Note you can run the tests like

cargo test -p datafusion-sql -- expr_to_sql_ok

Additional context

This is likely NOT a good first issue

[xinlifoobar]

Hi @alamb, I am wondering whether is there a way to implement the cube and 'rollup' functions easily. the inner vector is expanded in the planner.

[xinlifoobar]

I think from the Vec<Expr> could not mapping back to the only unique Vec<Vec<Expr>> and hence unparse the cube statement would be difficult. How about store another copy of origin Vec in the Cube and Rollup instance? i.e.,

pub enum GroupingSet {
    /// Rollup grouping sets
    Rollup(Vec<Expr>, Vec<Vec<Expr>>),
    /// Cube grouping sets
    Cube(Vec<Expr>, Vec<Vec<Expr>>),
    /// User-defined grouping sets
    GroupingSets(Vec<Vec<Expr>>),
}

[alamb]

Hi @xinlifoobar - another thing that we could potentially do is to work at this from the SQL statement level (rather than the Expr level)

As in implement a round trip test like this for a query with a grouping set:

https://github.com/apache/datafusion/blob/main/datafusion/sql/tests/sql_integration.rs#L4678-L4708

[xinlifoobar]

I made a mistake about the previous statements. The vector of cube and rollout is not expanded so the implement could be easy.

https://github.com/apache/datafusion/blob/main/datafusion/sql/src/expr/grouping_set.rs#L42-L81

[alamb]

Thanks again @xinlifoobar