dreverri opened a new pull request, #27333:
URL: https://github.com/apache/datafusion/pull/27333
## Which issue does this PR close?
- Closes #27332.
## Rationale for this change
Unparsing the optimized plan of `select count(*) from t where id > 10`
produces `SELECT count(1) AS "count(*)" FROM (SELECT FROM t WHERE (t.id >
10))`, even for dialects whose `supports_empty_select_list()` is `false`.
Engines that don't accept an empty select list, such as BigQuery ("SELECT list
must not be empty"), reject the generated SQL.
`optimize_projections` prunes the filter column above the `Filter`, which
leaves a `Projection` with no expressions. #19221 added the `SELECT 1` fallback
for empty projections, but only the `TableScan` path uses it.
## What changes are included in this PR?
- `reconstruct_select_statement` now uses `empty_projection_fallback()` when
the `Projection` has no expressions, the same as the `TableScan` path. Dialects
without empty select list support get `SELECT 1 FROM ...`. PostgreSQL output is
unchanged (`SELECT FROM ...`).
## Are these changes tested?
Yes.
- `datafusion/core/tests/sql/unparser.rs`:
`optimized_count_over_filter_empty_projection` runs the real query through the
optimizer and unparser. It fails without the fix.
- `datafusion/sql/tests/cases/plan_to_sql.rs`: snapshot tests for an empty
`Projection` node with the default, PostgreSQL and BigQuery dialects.
## Are there any user-facing changes?
Yes, generated SQL changes for dialects that don't support an empty select
list: an empty `Projection` now unparses as `SELECT 1 FROM ...` instead of
`SELECT FROM ...`. There are no public API changes.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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