harshmotw-db opened a new pull request, #47907:
URL: https://github.com/apache/spark/pull/47907
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### What changes were proposed in this pull request?
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This PR prohibits casts from data types containing structs or maps to
variant and introduces a new expression `to_variant_object` which allows
casting from nested types to variants and retains the old functionality. This
PR also changes the behavior of the `schema_of_variant` and
`schema_of_variant_agg` expressions where they now print `OBJECT` instead of
`STRUCT` (which is not technically correct).
### Why are the changes needed?
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Cast from structs to variant objects should not be legal since variant
objects are unordered bags of key-value pairs while structs are ordered sets of
elements of fixed types. Therefore, casts between structs and variant objects
do not behave like casts between structs. Example (produced by Serge Rielau):
```
scala> spark.sql("SELECT cast(named_struct('c', 1, 'b', '2') as struct<b
int, c int>)").show()
+------------------------+
|named_struct(c, 1, b, 2)|
+------------------------+
|{1, 2}|
+------------------------+
Passing a struct into VARIANT loses the position
scala> spark.sql("SELECT cast(named_struct('c', 1, 'b', '2')::variant as
struct<b int, c int>)").show()
+-----------------------------------------+
|CAST(named_struct(c, 1, b, 2) AS VARIANT)|
+-----------------------------------------+
|{2, 1}|
+-----------------------------------------+
```
Casts from maps to variant objects should also not be legal since they
represent completely orthogonal data types. Maps can represent a variable
number of key value pairs based on just a key and value type in the schema but
in objects, the schema (produced by schema_of_variant expressions) will have a
type corresponding to each value in the object. Objects can have values of
different types while maps cannot and objects can only have string keys while
maps can also have complex keys.
We should therefore prohibit the existing behavior of allowing explicit
casts from structs and maps to variants as the variant spec currently only
supports an object type which is remotely compatible with structs and maps. We
introduce a new expression that converts schemas containing structs and maps to
variants (where these types are converted to objects). We will call it
`to_variant_object`.
### Does this PR introduce _any_ user-facing change?
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Yes, it introduces the `to_variant_object` expression and changes the
behavior of the `schema_of_variant`/`schema_of_variant_agg` expressions.
### How was this patch tested?
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Several unit tests with codegen enabled/disabled.
### Was this patch authored or co-authored using generative AI tooling?
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Yes.
Generated-by: GitHub Copilot, perplexity.ai
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