Function union

prqlc/prqlc/src/sql/pq/preprocess.rs:277–303

Creates [SqlTransform::Union] from [Transform::Append]

(
    pipeline: Vec<SqlTransform>,
    ctx: &mut Context,
)

/// Creates [SqlTransform::Union] from [Transform::Append]
pub(in crate::sql) fn union(
    pipeline: Vec<SqlTransform>,
    ctx: &mut Context,
) -> Result<Vec<SqlTransform>> {
    use SqlTransform::*;
    use Transform::*;

    let mut res = Vec::with_capacity(pipeline.len());
    let mut pipeline = pipeline.into_iter().peekable();
    while let Some(t) = pipeline.next() {
        let Super(Append(bottom)) = t else {
            res.push(t);
            continue;
        };
        let bottom = ctx.anchor.create_relation_instance(bottom, HashMap::new());

        let distinct = if let Some(Distinct) = &pipeline.peek() {
            pipeline.next();
            true
        } else {
            false
        };

        res.push(SqlTransform::Union { bottom, distinct });
    }
    Ok(res)
}

/// Creates [SqlTransform::Except] from [Transform::Join] and [Transform::Filter]
pub(in crate::sql) fn except(