harshmotw-db opened a new pull request, #47907: URL: https://github.com/apache/spark/pull/47907
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If you want to add or modify an error type or message, please read the guideline first in 'common/utils/src/main/resources/error/README.md'. --> ### What changes were proposed in this pull request? <!-- Please clarify what changes you are proposing. The purpose of this section is to outline the changes and how this PR fixes the issue. If possible, please consider writing useful notes for better and faster reviews in your PR. See the examples below. 1. If you refactor some codes with changing classes, showing the class hierarchy will help reviewers. 2. If you fix some SQL features, you can provide some references of other DBMSes. 3. If there is design documentation, please add the link. 4. If there is a discussion in the mailing list, please add the link. --> 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? <!-- Please clarify why the changes are needed. For instance, 1. If you propose a new API, clarify the use case for a new API. 2. If you fix a bug, you can clarify why it is a bug. --> 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? <!-- Note that it means *any* user-facing change including all aspects such as the documentation fix. If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible. If possible, please also clarify if this is a user-facing change compared to the released Spark versions or within the unreleased branches such as master. If no, write 'No'. --> 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? <!-- If tests were added, say they were added here. Please make sure to add some test cases that check the changes thoroughly including negative and positive cases if possible. If it was tested in a way different from regular unit tests, please clarify how you tested step by step, ideally copy and paste-able, so that other reviewers can test and check, and descendants can verify in the future. If tests were not added, please describe why they were not added and/or why it was difficult to add. If benchmark tests were added, please run the benchmarks in GitHub Actions for the consistent environment, and the instructions could accord to: https://spark.apache.org/developer-tools.html#github-workflow-benchmarks. --> Several unit tests with codegen enabled/disabled. ### Was this patch authored or co-authored using generative AI tooling? <!-- If generative AI tooling has been used in the process of authoring this patch, please include the phrase: 'Generated-by: ' followed by the name of the tool and its version. If no, write 'No'. Please refer to the [ASF Generative Tooling Guidance](https://www.apache.org/legal/generative-tooling.html) for details. --> Yes. Generated-by: GitHub Copilot, perplexity.ai -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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