[I] INSERT into a partitioned table rejects NOT NULL source columns for optional table columns [datafusion-iceberg]

["NoahKusaba (via GitHub)" <gi...@apache.org>]

NoahKusaba opened a new issue, #27:
URL: https://github.com/apache/datafusion-iceberg/issues/27

   ### Describe the bug
   
   An `INSERT` into a **partitioned** table fails at planning time when a source column is `NOT NULL` and the table's column is optional:
   
   ```
   Error: Plan("Input schema does not match Iceberg table schema.
   Expected schema: Field { "id": nullable Int32 }, Field { "category": nullable Utf8 }
   Input schema: Field { "id": Int32 }, Field { "category": Utf8 }")
   ```
   
   Every value of a non-nullable column is valid in an optional one, so the write is safe. `project_with_partition` compares the input and table schemas with `==`, which also requires equal nullability. It only runs for partitioned tables, so the same `INSERT` into an unpartitioned table succeeds.
   
   ### To Reproduce
   
   1. Create a table with optional columns `id: int` and `category: string`, partitioned by identity on `category`.
   2. Register a `MemTable` source whose `id` and `category` columns are non-nullable.
   3. `INSERT INTO t SELECT * FROM source` fails with the error above.
   
   A `VALUES` list with no NULLs fails the same way, since DataFusion infers its columns as non-nullable.
   
   ### Expected behavior
   
   The `INSERT` succeeds. Only a nullable input column into a required table column should be rejected.
   
   Related: #22 covers the opposite direction. Part of #23.
   


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