fixfon opened a new pull request, #26722:
URL: https://github.com/apache/datafusion/pull/26722
## Which issue does this PR close?
- Closes #26316.
## Rationale for this change
The unparser turns a NULL list literal into `[]`, so the SQL it writes is a
different value from the plan (a NULL `FixedSizeList` came out as `[NULL,
NULL]`). A NULL struct, map or union literal fails with
`NotImplemented("Unsupported scalar")`, so a plan that contains one can't be
unparsed at all.
## What changes are included in this PR?
`scalar_to_sql` now catches a NULL list, struct, map or union first and
writes `NULL`, the same bare `NULL` the other typed nulls already use. Non-null
values are handled as before, so a non-null struct, map or union still returns
`NotImplemented`.
## What is the testing strategy for this PR?
Two unit tests in `unparser/expr.rs`, through `expr_to_sql` (sqllogictest
can't reach the unparser). One checks that a NULL of each of the eight types
gives `NULL`, and that a list holding a NULL list gives `[NULL, [1]]`. The
other checks that an empty list is still `[]` and a non-null struct still
returns `NotImplemented`. The first test fails on main with eight wrong results
and passes with the change. `cargo test -p datafusion-sql` passes.
Not covered: a union whose active value is NULL, a NULL list wrapped in a
dictionary, and non-default dialects. The bare `NULL` has no type, so a target
engine may infer a different one.
## Are there any user-facing changes?
Unparsed SQL now has `NULL` where it used to have `[]`, or an error. No API
changes.
AI assistance: Claude (via T3 Code) helped write the change and the test.
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