Re: [I] GROUP BY constant with aggregation function and with empty input magically summons a row [datafusion]

["viirya (via GitHub)" <gi...@apache.org>]

viirya commented on issue #15734:
URL: https://github.com/apache/datafusion/issues/15734#issuecomment-5387167357

   This appears to be fixed on current `main` — I can no longer reproduce it.
   
   Both variants from the report now return zero rows:
   
   ```sql
   SELECT 'wat' AS key, count(1) AS c FROM (SELECT * FROM unnest(array[]::bigint[])) AS b(field) GROUP BY key;
   -- 0 rows
   ```
   
   and the same against a registered empty table. The optimized plan keeps the grouping rather than dropping it:
   
   ```
   Aggregate: groupBy=[[Utf8("wat")]], aggr=[[count(Int64(1))]]
   ```
   
   The fix looks to be #22132 ("fix: eliminate group by constant empty input", merged 2026-07-27), which matches the root cause @Dandandan identified here — `EliminateGroupByConstant` now declines to eliminate constant GROUP BY expressions when doing so would remove *all* of the grouping expressions:
   
   ```rust
   if redundant.is_empty() || required.is_empty() {
       return Ok(Transformed::no(LogicalPlan::Aggregate(aggregate)));
   }
   ```
   
   That PR closed #11748 but not this one, so this issue looks safe to close as already fixed.
   


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