fix: Do not add redundant exchange over remote function

[feilong-liu]

Description

We added an option in #27044 to set the number of tasks for remote functions, however we observe that it will add an unnecessary exchange even when the remote function directly send data to output node. This PR will remove this unnecessary exchange node.

Motivation and Context

Simplify plans

Impact

Make the plan to streaming result out.

Test Plan

Unit tests

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Release Notes

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== NO RELEASE NOTE ==

Summary by Sourcery

Adjust exchange planning for remote function projections to avoid adding unnecessary trailing exchanges when the plan is not required to be distributed.

Bug Fixes:

• Prevent insertion of a redundant trailing remote exchange for fixed-parallelism remote function projections when the preferred global properties are undistributed.

Tests:

• Update existing remote function fixed-parallelism plan tests to expect a gather exchange at the root where appropriate.
• Add tests covering when trailing exchanges are skipped or preserved for remote function projections under undistributed vs distributed plans.

[sourcery-ai]

Reviewer's Guide

Updates AddExchanges so it only adds a trailing remote round‑robin exchange for remote fixed‑parallelism projections when the preferred global properties are distributed, and adjusts/extends planner tests to assert the new behavior for both distributed and undistributed cases.

Sequence diagram for AddExchanges conditional trailing exchange on ProjectNode

sequenceDiagram
    participant Planner as Planner
    participant AddExchanges as AddExchanges
    participant PreferredProperties as PreferredProperties
    participant GlobalProps as GlobalProperties

    Planner->>AddExchanges: visitProject(projectNode, preferredProperties)
    AddExchanges->>AddExchanges: create first remote roundRobinExchange
    AddExchanges->>AddExchanges: replace ProjectNode child with first exchange

    AddExchanges->>PreferredProperties: getGlobalProperties()
    PreferredProperties-->>AddExchanges: Optional<GlobalProperties>
    AddExchanges->>GlobalProps: isDistributed()
    GlobalProps-->>AddExchanges: boolean

    alt distributed or absent (default true)
        AddExchanges->>AddExchanges: create trailing remote roundRobinExchange
        AddExchanges-->>Planner: PlanWithProperties(ProjectNode + two exchanges)
    else undistributed
        AddExchanges-->>Planner: PlanWithProperties(ProjectNode + one exchange)
    end

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File-Level Changes

Change	Details	Files
Gate the addition of the final remote round-robin exchange behind a distributed-preference check in AddExchanges.	After building the fixed-parallelism remote round-robin exchange for a ProjectNode, only add the final REMOTE_STREAMING round-robin exchange when PreferredProperties.global is distributed or unspecified (default true). Leave the existing behavior unchanged for distributed plans while skipping the extra exchange for undistributed plans, which makes the output stage undistributed in those cases.	presto-main-base/src/main/java/com/facebook/presto/sql/planner/optimizations/AddExchanges.java
Align and extend AddExchanges planner tests to the new exchange behavior for remote functions with fixed parallelism.	Change expected top-level exchange in several existing remote fixed-parallelism tests from REMOTE_STREAMING REPARTITION to REMOTE_STREAMING GATHER, reflecting the removal of the extra repartitioning exchange at the root. Add a test that verifies the trailing exchange is skipped when the plan is undistributed and both LIMIT and non-LIMIT queries use a GATHER root exchange over a remote projection. Add a complementary test ensuring the trailing exchange is not skipped for distributed queries (e.g., DISTINCT + LIMIT) and the plan still includes a REPARTITION exchange above the remote projection. Keep other test scaffolding (session properties, projections, scans) consistent so only the shape and distribution of the exchange nodes change.	presto-main-base/src/test/java/com/facebook/presto/sql/planner/optimizations/TestAddExchangesPlansWithFunctions.java

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[sourcery-ai]

Hey - I've left some high level feedback:

• In AddExchanges.visitProject, the new condition uses preferredProperties.getGlobalProperties().map(PreferredProperties.Global::isDistributed).orElse(true); consider whether orElse(true) is really desired (i.e., defaulting to adding the trailing exchange when distribution is unknown) or if defaulting to false would better reflect the intended behavior.
• The two new tests for skipping vs not skipping the trailing exchange build the same session configuration multiple times; consider extracting a small helper to construct that session to keep the tests more focused on the query shape and expected plan.
• The method name testRemoteFunctionFixedParallelismNotSkipsTrailingExchangeWhenDistributed is a bit hard to parse; renaming to something like testRemoteFunctionFixedParallelismKeepsTrailingExchangeWhenDistributed would make the intent clearer.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `AddExchanges.visitProject`, the new condition uses `preferredProperties.getGlobalProperties().map(PreferredProperties.Global::isDistributed).orElse(true)`; consider whether `orElse(true)` is really desired (i.e., defaulting to adding the trailing exchange when distribution is unknown) or if defaulting to `false` would better reflect the intended behavior.
- The two new tests for skipping vs not skipping the trailing exchange build the same session configuration multiple times; consider extracting a small helper to construct that session to keep the tests more focused on the query shape and expected plan.
- The method name `testRemoteFunctionFixedParallelismNotSkipsTrailingExchangeWhenDistributed` is a bit hard to parse; renaming to something like `testRemoteFunctionFixedParallelismKeepsTrailingExchangeWhenDistributed` would make the intent clearer.

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[kaikalur]

Actually, it should never be needed because if the next stage needs exchange, it will be done automatically.

[feilong-liu]

Actually, it should never be needed because if the next stage needs exchange, it will be done automatically.

You are right, we do not need this additional exchange. Updated the code