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?
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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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