Function prune_inputs

prqlc/prqlc/src/sql/pq/preprocess.rs:59–94

Removes unused relation inputs

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

/// Removes unused relation inputs
pub(in crate::sql) fn prune_inputs(
    mut pipeline: Vec<SqlTransform>,
    ctx: &mut Context,
) -> Result<Vec<SqlTransform>> {
    use SqlTransform::Super;

    let mut used_cids = HashSet::new();

    let mut res = Vec::new();
    while let Some(mut transform) = pipeline.pop() {
        // collect cids (special case for Join & From)
        match transform {
            SqlTransform::Join { ref filter, .. } => {
                used_cids.extend(CidCollector::collect(filter.clone()));
            }
            SqlTransform::From(_) => {}
            Super(t) => {
                let (t, cids) = CidCollector::collect_t(t);
                used_cids.extend(cids);
                transform = Super(t);
            }
            _ => unreachable!(),
        }

        // prune unused inputs
        if let SqlTransform::From(with) | SqlTransform::Join { with, .. } = &mut transform {
            let relation = ctx.anchor.relation_instances.get_mut(with).unwrap();
            (relation.table_ref.columns).retain(|(_, cid)| used_cids.contains(cid));
        }

        res.push(transform);
    }

    res.reverse();
    Ok(res)
}

pub(in crate::sql) fn wrap(pipe: Vec<Transform>, ctx: &mut Context) -> Result<Vec<SqlTransform>> {
    // We map From and Join into SqlTransforms, because we need to change their RIIds.