Function attempt_outer_equijoin

fn attempt_outer_equijoin(
    left: MirRelationExpr,
    right: MirRelationExpr,
    on: MirScalarExpr,
    on_subquery_types: Vec<SqlColumnType>,
    kind: JoinKind,
    oa: usize,
    id_gen: &mut IdGen,
    context: &Context<'_>,
) -> Result<Option<MirRelationExpr>, PlanError>

Attempts an efficient outer join, if on has equijoin structure.

Both left and right are decorrelated inputs.

The first oa columns correspond to an outer context: we should do the outer join independently for each prefix. In the case that on contains just some equality tests between columns of left and right and some local predicates, we can employ a relatively simple plan.

The last on_subquery_types.len() columns correspond to results from subqueries defined in the on clause - we treat those as theta-join conditions that prohibit the use of the simple plan attempted here.