cloud-fan commented on a change in pull request #29311:
URL: https://github.com/apache/spark/pull/29311#discussion_r465488975
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File path: docs/sql-migration-guide.md
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@@ -34,6 +34,8 @@ license: |
- In Spark 3.1, structs and maps are wrapped by the `{}` brackets in casting
them to strings. For instance, the `show()` action and the `CAST` expression
use such brackets. In Spark 3.0 and earlier, the `[]` brackets are used for the
same purpose. To restore the behavior before Spark 3.1, you can set
`spark.sql.legacy.castComplexTypesToString.enabled` to `true`.
+ - In Spark 3.1, `CAST` converts NULL elements of structures, arrays and maps
to "null". In Spark 3.0 or earlier, NULL elements are converted to empty
strings. To restore the behavior before Spark 3.1, you can set
`spark.sql.legacy.castComplexTypesToString.enabled` to `true`.
Review comment:
`In Spark 3.1, NULL elements of structures, arrays and maps are
converted to "null" in casting them to strings.`
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