Function expand_rollup

nodedb-sql/src/planner/grouping_sets.rs:158–164

Expand `ROLLUP(a, b, c)` → `[[a,b,c], [a,b], [a], []]`. The input is `Vec >` where each inner vec is one composite element. We flatten composite elements to individual expressions for simplicity — the outer product is: suffix-strip from all-present down to empty.

(groups: &[Vec<ast::Expr>])

/// We flatten composite elements to individual expressions for simplicity — the
/// outer product is: suffix-strip from all-present down to empty.
fn expand_rollup(groups: &[Vec<ast::Expr>]) -> Vec<Vec<&ast::Expr>> {
    // Flatten composite groups (e.g. `(a, b)` as one element) into atoms.
    let atoms: Vec<&ast::Expr> = groups.iter().flat_map(|g| g.iter()).collect();
    let n = atoms.len();
    // Prefixes: atoms[0..n], atoms[0..n-1], ..., atoms[0..0] (empty).
    (0..=n).rev().map(|len| atoms[..len].to_vec()).collect()
}

/// Expand `CUBE(a, b)` → all 2^N subsets.
fn expand_cube(groups: &[Vec<ast::Expr>]) -> Vec<Vec<&ast::Expr>> {