Module subquery 

Expression-level subquery execution.

EXISTS / IN subqueries at non-conjunctive expression positions (WHERE residual, RETURN, HAVING, WITH assignments, …) are compiled at planning time by plan_expression_subqueries into standalone sub-plans. Each compiled subquery becomes a SubqueryRunnerSpec on the hosting Filter/Project/Assign arena spec; the materializer instantiates a per-operator SubqueryExecutor from those specs.

Execution model:

• Every operator instance owns exactly one SubqueryExecutor; parallel partitions never share mutable runner state.
• A runner materializes its sub-plan once on first use and re-runs it per row via the reset protocol (StreamingExecutor::reset), never rebuilding the pipeline.
• Correlated runners receive the current row as a private correlation frame (StreamingExecutor::inject_correlation_frame); the frame is injected into the sub-plan’s Argument source slot.
• Non-correlated runners evaluate once and cache the result (EXISTS → bool, IN → HashSet<Value> with NULLs removed).

NULL semantics: a NULL left operand or NULL result values never match, consistent with the conjunctive keys_match path.

Structs

EvalEnv

Bundle of per-evaluation-environment state threaded into batch expression evaluation. Replaces the old standalone params argument so future operator kinds only need to add their facility to the env.

GroupJoinSpec

Group-Join decorrelation info for a scalar aggregate subquery.

SubqueryExecutor

Per-operator subquery execution facility.

SubqueryRunner

A compiled subquery with a reusable, resettable executor instance.

SubqueryRunnerSpec

Immutable subquery config carried by the arena spec.