Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/20224#discussion_r163449646
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
---
@@ -365,6 +388,26 @@ case class WholeStageCodegenExec(child: SparkPlan)
extends UnaryExecNode with Co
"pipelineTime" -> SQLMetrics.createTimingMetric(sparkContext,
WholeStageCodegenExec.PIPELINE_DURATION_METRIC))
+ /**
+ * ID for codegen stages within a query plan.
+ * It does not affect equality, nor does it participate in destructuring
pattern matching.
+ *
+ * Within a query, a codegen stage in a plan starts counting from 1, in
"insertion order".
+ * WholeStageCodegenExec operators are inserted into a plan in
depth-first post-order.
+ * See CollapseCodegenStages.insertWholeStageCodegen for the definition
of insertion order.
+ *
+ * 0 is reserved as a special ID value to indicate a temporary
WholeStageCodegenExec object
+ * is created, e.g. for special fallback handling when an existing
WholeStageCodegenExec
+ * failed to generate/compile code.
+ */
+ val codegenStageId = WholeStageCodegenId.getNextStageId()
--- End diff --
my major concern is, what if we transform the physical plan tree after
adding `WholeStageCodegenExec`? e.g. during transformation we may copy a plan
code, then a copied `WholeStageCodegenExec` will have a different stage id.
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