pan3793 opened a new pull request, #41070:
URL: https://github.com/apache/spark/pull/41070
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### What changes were proposed in this pull request?
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Add a new method `createTableReserveSchemaNullability` in `Table`, to allow
the DataSource implementation to declare whether they need to reserve schema
nullability on CTAS/RTAS.
### Why are the changes needed?
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SPARK-28837 forcibly uses the nullable schema on CTAS/RTAS, which seems too
aggressive:
1. The existing matured RDBMSs have different behaviors for reserving schema
nullability on CTAS/RTAS, as mentioned in #25536, PostgreSQL forcibly uses the
nullable schema, but MySQL respects the query's output schema nullability.
2. Some OLAP systems(e.g. ClickHouse) are perf-sensitive for nullable, and
have strict restrictions on table schema, e.g. the primary keys are not allowed
to be nullable.
### Does this PR introduce _any_ user-facing change?
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Yes, this PR adds a new to DSv2 API, but the default implementation reserves
the backward compatibility.
### How was this patch tested?
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UTs are updated.
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