Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/20224#discussion_r163770850
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
---
@@ -325,6 +326,32 @@ object WholeStageCodegenExec {
}
}
+object WholeStageCodegenId {
+ // codegenStageId: 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.
+
--- End diff --
I think we should describe about the usage of such codegen stage id, e.g.,
the codegen stage id would show up in explain string and generated class name.
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