Altering a MULTISET column's element type fails with a misleading ColumnNotExistException

[LuciferYang]

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Paimon version

master (1.5-SNAPSHOT)

Compute Engine

Any engine issuing ALTER TABLE on a MULTISET column.

Minimal reproduce step

1. Create a table with a MULTISET column, e.g. tags MULTISET<INT>.
2. Alter the multiset element type, e.g. to MULTISET<BIGINT> (or change the element nullability).

What doesn't meet your expectations?

The ALTER fails with a misleading ColumnNotExistException, even though the column exists. The nested type-update walk has no MULTISET case, so an off-by-one routes the change to updateLastColumn on an empty list. A MULTISET<T> is physically stored as InternalMap<T, Int>, so the element is a map key, and the read-side cast infrastructure has no MULTISET executor and deliberately forbids changing a map key type. So a multiset element type change cannot be applied to existing data and should be rejected at DDL with a clear, actionable message rather than a confusing "column not exist".

Anything else?

The MULTISET column's own top-level nullability change (the column, not its element) is a separate, supported operation and should keep working.

Are you willing to submit a PR?

• I'm willing to submit a PR!