feat(physical-plan): expose finalized Accumulator state on BoundedWindowAggExec - #24031
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…dowAggExec Per-output-partition slots hold each aggregate window function's final Accumulator::state() at partition-close, plus a getter that operators can call once a task has drained. Intended to support prefix scans over ordered window aggregates. The stream snapshots state at the top of prune_state — the last moment state.is_end entries are live before either prune_out_columns or prune_partition_batches drops them — so both mid-stream partition close (as SQL PARTITION BY groups end) and EOS are covered by one write site. Non-aggregate window functions (row_number, rank, lead/lag, ...) occupy None at their slot. Storage is Arc<[OnceLock<Vec<Option<Vec<ScalarValue>>>>]> — one slot per output partition, first-close-wins. The getter is scoped to the at-most-one-PARTITION-BY-group case; additional groups are silently dropped rather than merged (no well-defined semantics for combining). No trait changes: uses the existing Accumulator::state() surface already relied on by the group-by Partial/Final protocol, exposed through a new method on the already-public WindowState so no internal enum leaks into the public API. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Adds per-output-partition slots on
BoundedWindowAggExecthat capture each aggregate window function's finalAccumulator::state()at partition-close, plus a getter (finalized_partition_state) for downstream operators to read once a task has drained. Intended to support prefix scans over ordered window aggregates.Slim re-do of #24007 addressing the review there. Public surface is a single new
pub fnon BWAG plus one method on the already-publicWindowState; no new pub types or re-exports.What changed vs. #24007
pub use window_expr::WindowFnleaked internal enum withBox<dyn Accumulator>/Box<dyn PartitionEvaluator>variantsWindowState::aggregate_state() -> Result<Option<Vec<ScalarValue>>>on the already-public type.pub type FinalizedPartitionStatealiastype_complexity.Empty/Single/Multistate machine + out-of-range error + mutex-poison errorOption<&[Option<Vec<ScalarValue>>]>.Arc<[Mutex<FinalStateSlot>]>Arc<[OnceLock<Vec<Option<Vec<ScalarValue>>>>]>— no Mutex, no state machine, first-close-wins.Diff: 429 → 168 lines.
Test plan
cargo test -p datafusion-physical-plan --lib windows::bounded_window_agg_exec::tests::finalized_partition_state_returns_single_group_statecargo test -p datafusion-physical-plan --lib windows(11 passed)cargo clippy -p datafusion-physical-plan -p datafusion-physical-expr --all-targets(clean)cargo fmt --all --check