Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/23045#discussion_r233873941
--- Diff: docs/sql-migration-guide-upgrade.md ---
@@ -17,6 +17,8 @@ displayTitle: Spark SQL Upgrading Guide
- The `ADD JAR` command previously returned a result set with the single
value 0. It now returns an empty result set.
+ - In Spark version 2.4 and earlier, users can create map values with map
type key via built-in function like `CreateMap`, `MapFromArrays`, etc. Since
Spark 3.0, it's not allowed to create map values with map type key.
+
--- End diff --
I think should we also add this note?
> Note that, maps with map type key still exist, via reading from parquet
files, converting from scala/java map, etc. Spark is not to completely forbid
map as map key, but to avoid creating it by Spark itself.
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