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Posted to reviews@spark.apache.org by "harshmotw-db (via GitHub)" <gi...@apache.org> on 2025/03/20 22:28:12 UTC

[PR] [SPARK-51576] Prohibit ANSI cast from `array` to `array<_, containsNull = false>` [spark]

harshmotw-db opened a new pull request, #50343:
URL: https://github.com/apache/spark/pull/50343

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   ### What changes were proposed in this pull request?
   
   This PR prohibits ANSI cast from `ArrayType(VariantType, containsNull = false)` to `ArrayType(_, containsNull = false)`, or `StructField(VariantType, nullable = false)` to `StructField(_, nullable = true)` or `MapType(_, VariantType, valueContainsNull = false)` to `MapType(_, _, valueContainsNull = false)`.
   
   ### Why are the changes needed?
   <!--
   Please clarify why the changes are needed. For instance,
     1. If you propose a new API, clarify the use case for a new API.
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   Variant is the only data type which can be ANSI cast from a non-null value to a null value. More specifically, variant nulls (generated using `parse_json('null')`) when cast to string result in `null`.
   However, `array<variant, containsNull = false>` can be cast to `array<string, containsNull = true>`. This should not be allowed since `array(parse_json('null'))` would give `array(null)` after this cast.
   Here is a demonstration of this problem where we create an `array<string, containsNull = true>` which in fact does not contain nulls:
   ```
   >>> from pyspark.sql.functions import col
   >>> from pyspark.sql.types import StringType, VariantType, ArrayType
   >>> 
   >>> df = spark.sql("select array(parse_json('null')) arr")
   >>> df.printSchema()
   root
    |-- arr: array (nullable = false)
    |    |-- element: variant (containsNull = false)
   
   >>> df2 = df.select(col('arr').cast(ArrayType(StringType(), False)))
   >>> df2.selectExpr("arr[0] is null").show()
   +----------------+
   |(arr[0] IS NULL)|
   +----------------+
   |            true|
   +----------------+
   
   >>> df2.printSchema()
   root
    |-- arr: array (nullable = false)
    |    |-- element: string (containsNull = false)
   ```
   
   
   ### Does this PR introduce _any_ user-facing change?
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   Yes, users can no longer ANSI cast from `array<variant, containsNull = false>` to `array<_, containsNull = false>`. However, it is unlikely that users were hardcoding `containsNull = false`.
   
   ### How was this patch tested?
   
   Unit tests making sure that only `containsNull = false` cases fail.
   
   ### Was this patch authored or co-authored using generative AI tooling?
   
   No.


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