Function infer_subquery_relation 

pub fn infer_subquery_relation(
    catalog: &dyn SqlCatalog,
    alias: &str,
    query: &Query,
    plan: Option<&SqlPlan>,
    functions: &FunctionRegistry,
    temporal: TemporalScope,
) -> Result<CollectionInfo>

The relation a subquery alias exposes.

A synthesized relation carries EngineType::DocumentSchemaless: it is a MessagePack row stream, and the CTE lowering depends on that. Openness rides on CollectionInfo::open_schema, not on the engine.

plan is the subquery’s own plan. The search cells (distance, _surrogate) are read from it, never re-derived from the AST: the planner is the only layer that decides whether an operator form routes to a search. A correlated LATERAL subquery cannot be planned at scope time and carries no plan: it routes no search cells either (the lateral planner handles its shapes directly).