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// SPDX-License-Identifier: Apache-2.0
//! Exchange is the single coordinator-mediated data-movement operator in the
//! physical plan tree. It is resolved by the Control-Plane coordinator and
//! NEVER executed on a Data-Plane core.
//!
//! - `Gather` fans the child plan out to all cores and merges their results
//! back on the coordinator. When `as_aggregate` is true the merge is an
//! aggregate reduction; otherwise it is a plain concatenation.
//! - `Broadcast` gathers the child plan to the coordinator so that its result
//! can be embedded as an inline input into a sibling operator (e.g. the
//! build side of a `HashJoin`).
//! - `Shuffle` wraps a complete `HashJoin` at the plan root and drives a
//! cross-node hash-repartition grace join: the coordinator resolver fans
//! per-side scan producers to each collection's owner, repartitions rows on
//! the join keys to part-owners, runs the node-local grace join on each part,
//! and merges the results. It is coordinator-resolved and NEVER reaches a
//! core.
/// Data-movement node; coordinator-resolved, never reaches a core.
/// Movement strategy for an [`ExchangeOp`].