924060929 commented on code in PR #63366:
URL: https://github.com/apache/doris/pull/63366#discussion_r3413654832
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fe/fe-core/src/main/java/org/apache/doris/nereids/NereidsPlanner.java:
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@@ -740,6 +742,19 @@ protected void distribute(PhysicalPlan physicalPlan,
ExplainLevel explainLevel)
splitFragments(physicalPlan);
doDistribute(canUseNereidsDistributePlanner, explainLevel);
+
+ addLocalExchangeAfterDistribute();
Review Comment:
> @morrySnow just curious, why don't you impl it before translation and
after post process in Nereids?
(Answering the review-summary question here, anchored to where it actually
runs.)
It needs two inputs that only exist **after translation + distribute**, not
on the Nereids physical plan:
1. **PlanNode-level info** — `AddLocalExchange` walks the translated
`PlanNode` / `PlanFragment` tree via `PlanNode.enforceAndDeriveLocalExchange`
(`isSerialNode()` / `isSerialOperatorOnBe()`, the distribute-expr lists
serialized to BE thrift, the fragment sink type, …). Those are the legacy
`PlanNode`s the translator produces — they don't exist on the Nereids physical
operators.
2. **Instance counts** — whether to insert an LE (and the fan-out width)
depends on the per-BE instance count: `addLocalExchange` skips fragments where
`maxPerBeInstances <= 1` (mirroring BE's `_num_instances <= 1` no-op), and the
PASSTHROUGH fan-out targets `_num_instances`. Instance assignment is done by
`DistributePlanner`, which runs **after** translation.
So it runs right here — after `splitFragments()` + `doDistribute()` — the
first point where both the BE-facing PlanNode tree and the instance assignment
exist. Doing it pre-translation in Nereids would mean re-deriving fragment
structure + instance counts that haven't been computed yet.
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