shahwazreza opened a new pull request, #27000:
URL: https://github.com/apache/datafusion/pull/27000
## Which issue does this PR close?
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- Closes #26431
## Rationale for this change
`SELECT ... ORDER BY ... LIMIT 0` panics instead of returning an empty
result when the limit makes it to the physical plan. Normally `eliminate_limit`
removes `LIMIT 0` before that happens, so most people won't run into it. But it
does reach execution with `datafusion.optimizer.max_passes = 0`, or when
someone builds the physical plan directly. In that case `SortExec` with `fetch
= Some(0)` builds a `TopK` with `k = 0`, and `TopKHeap::new` hits `assert!(k >
0)`.
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## What changes are included in this PR?
- `SortExec::execute` handles `fetch = Some(0)` on unsorted input before it
gets to the TopK path, and returns an empty stream.
- That partition still gets marked as emitted on the shared TopK dynamic
filter. Without this, the filter would wait forever for a partition that never
reports in. `mark_topk_emitted` is now `pub(crate)` so `SortExec` can call it.
I left the `assert!(k > 0)` in `TopKHeap` alone. Handling the case in
`SortExec` seemed cleaner than making TopK deal with an empty heap.
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## What is the testing strategy for this PR?
- Unit test `test_sort_fetch_zero_returns_empty_and_completes_filter` in
`sort.rs`. It runs a two-partition `SortExec` with `fetch = Some(0)` and checks
that both partitions return no rows. It also checks that the dynamic filter
completes only after the last partition has run.
- sqllogictest case in `limit.slt` that sets `max_passes = 0`, so the `LIMIT
0` survives into the physical plan. It checks the `EXPLAIN` output (`SortExec:
TopK(fetch=0)`) and that the query returns no rows.
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## Are there any user-facing changes?
The query now returns an empty result instead of panicking.
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