Nested `unnest` of a struct reports an internal error ("Assertion failed: struct_allowed") instead of a planning error

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
85/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
databases

Research direction

Start in datafusion/sql/src/utils.rs at RecursiveUnnestRewriter::transform, where struct_allowed currently produces the internal error. Review the unnest.slt coverage, including the arrow_typeof(unnest(column5)) case, and run the relevant SQL tests. Done means nested struct unnest expressions report the stated planning error instead of an internal error.

Written by the indexing model from the issue text.

Description

Describe the bug

unnest on a struct is only supported as the top-level select expression. Using it anywhere else hits assert_or_internal_err!(struct_allowed, ...) in RecursiveUnnestRewriter::transform (datafusion/sql/src/utils.rs), so the user gets an internal error that asks them to file a bug, even though the condition is entirely determined by the shape of their query.

To Reproduce
SELECT unnest(unnest(struct([1])));
SELECT unnest(struct(1))['c0'];

Internal error: Assertion failed: struct_allowed: unnest on struct can only be applied at the root level of select expression.
This issue was likely caused by a bug in DataFusion's code. Please help us to resolve this by filing a bug report in our issue tracker: https://github.com/apache/datafusion/issues

unnest.slt even has a test (select arrow_typeof(unnest(column5)) from unnest_table) that pins this internal-error text.

Expected behavior
Error during planning: unnest on struct can only be applied at the root level of select expression
Additional context

Found while running a corpus of unusual unnest expressions against datafusion-cli.

Dominant language
Rust
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