abjain123 opened a new issue, #18305:
URL: https://github.com/apache/iceberg/issues/18305
### Apache Iceberg version
1.11.0 (latest release)
### Query engine
Spark
### Please describe the bug 🐞
Iceberg v3 introduces the unknown data type to gracefully represent columns
without physical storage (such as Spark's VOID / all-null columns). However,
attempting to lazily evolve an unknown column into a concrete primitive type
(e.g., STRING) using Spark SQL's _ALTER TABLE ... ALTER COLUMN ... TYPE_
statement fails during logical plan execution.
TypePromotion.canPromote() in Iceberg currently does not handle promotions
from UnknownType to primitive types during schema evolution validation,
resulting in an IllegalArgumentException
**_Steps to Reproduce:_**
```
spark.sql("CREATE DATABASE IF NOT EXISTS ds2_standalone.iceberg_v3_qe")
spark.sql("DROP TABLE IF EXISTS ds2_standalone.iceberg_v3_qe.table1")
spark.sql("CREATE TABLE ds2_standalone.iceberg_v3_qe.table1 (id INT, payload
VOID) USING ICEBERG TBLPROPERTIES ('format-version'='3')")
spark.sql("INSERT INTO ds2_standalone.iceberg_v3_qe.table1 (id) VALUES (1),
(2)")
spark.sql("SELECT payload FROM ds2_standalone.iceberg_v3_qe.table1").show()
spark.sql("ALTER TABLE ds2_standalone.iceberg_v3_qe.table1 ALTER COLUMN
payload TYPE STRING") ## Fails here!
```
**_Error_**:
`The job crashes during logical plan execution with:
java.lang.IllegalArgumentException: Unsupported table change: Cannot change
column type: payload: unknown -> string`
### Willingness to contribute
- [ ] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from
the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time
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