Function enforce_linear_chains

src/expr/src/explain.rs:223–279

Normalize the way inputs of multi-input variants are rendered. After the transform is applied, non-trival inputs `$input` of variants with more than one input are wrapped in a `let $x = $input in $x` blocks. If these blocks are subsequently pulled up by `NormalizeLets`, the rendered version of the resulting tree will only have linear chains.

(expr: &mut MirRelationExpr)

/// If these blocks are subsequently pulled up by `NormalizeLets`,
/// the rendered version of the resulting tree will only have linear chains.
pub fn enforce_linear_chains(expr: &mut MirRelationExpr) -> Result<(), ExplainError> {
    use MirRelationExpr::{Constant, Get, Join, Union};

    if expr.is_recursive() {
        // `linear_chains` is not implemented for WMR, see
        // https://github.com/MaterializeInc/database-issues/issues/5631
        return Err(LinearChainsPlusRecursive);
    }

    // helper struct: a generator of fresh local ids
    let mut id_gen = id_gen(expr).peekable();

    let mut wrap_in_let = |input: &mut MirRelationExpr| {
        match input {
            Constant { .. } | Get { .. } => (),
            input => {
                // generate fresh local id
                // let id = id_cnt
                //     .next()
                //     .map(|id| LocalId::new(1000_u64 + u64::cast_from(id_map.len()) + id))
                //     .unwrap();
                let id = id_gen.next().unwrap();
                let value = input.take_safely(None);
                // generate a `let $fresh_id = $body in $fresh_id` to replace this input
                let mut binding = MirRelationExpr::Let {
                    id,
                    value: Box::new(value),
                    body: Box::new(Get {
                        id: Id::Local(id.clone()),
                        typ: input.typ(),
                        access_strategy: AccessStrategy::UnknownOrLocal,
                    }),
                };
                // swap the current body with the replacement
                std::mem::swap(input, &mut binding);
            }
        }
    };

    expr.try_visit_mut_post(&mut |expr: &mut MirRelationExpr| {
        match expr {
            Join { inputs, .. } => {
                for input in inputs {
                    wrap_in_let(input);
                }
            }
            Union { base, inputs } => {
                wrap_in_let(base);
                for input in inputs {
                    wrap_in_let(input);
                }
            }
            _ => (),
        }
        Ok(())
    })
}