Integrity constraints in DFSchema

[RadimBaca]

I'm writing a rewriting rule, and I need information about integrity constraints such as primary key or foreign key.

    let sql_query = "
    select p.*, CONCAT(first_name, ' ', last_name)
    from people p
    join department d on d.did = p.did
    where first_name = 'Jiri' and last_name = 'Prochazka';";

    // first set up the config
    let mut config = SessionConfig::default();
    let ctx = SessionContext::new_with_config(config);

    ctx.sql("CREATE TABLE department (\
        did INT PRIMARY KEY,\
        name VARCHAR(50), \
        year_started INT, \
        year_ended INT   \
    )").await?.collect().await?;

    ctx.sql("CREATE TABLE people ( \
        pid INT PRIMARY KEY, \
        first_name VARCHAR(50),  \
        last_name VARCHAR(50), \
        did INT NOT NULL  \
    )").await?.collect().await?;

    let logical_plan = ctx.sql(sql_query).await?.into_optimized_plan()?;

I considered adding Constraints struct into DFSchema since DFSchema is available in LogicalPlan. Another change I need is that the Foreign Key variant has to be added to the Constraint enum. Is there any better solution to this?