This is an extension of SPARK-22543 to fix 64kb compile error for deeply nested expressions under wholestage codegen.
Split codegen'd aggregation code into small functions for the HotSpot
GeneratedIteratorForCodegenStage1 grows beyond 64 KB
[Github] Pull Request #19813 (viirya)
[Github] Pull Request #19969 (maropu)
[Github] Pull Request #21140 (viirya)