jayzhan211 opened a new pull request, #24958:
URL: https://github.com/apache/datafusion/pull/24958
## Which issue does this PR close?
- Closes #.
## Rationale for this change
`ORDER BY` applied on top of a `LIMIT ... OFFSET` subquery returns too few
rows.
```sql
SELECT * FROM (SELECT a FROM t1 LIMIT 4 OFFSET 3) ORDER BY a;
```
`EnforceSorting` pushes the sort below the `GlobalLimitExec` and converts it
into a
TopK. The TopK's fetch was seeded from `GlobalLimitExec::fetch()`, which is
the
limit's *output* row count and ignores `skip`. So the TopK kept only 4 rows,
the
limit then skipped 3 of them, and the query returned a single row instead of
4.
The same seeding happens in two places in `sort_pushdown.rs`: when a root's
direct
child is a `GlobalLimitExec` (`assign_initial_requirements`) and when the
pushdown
descends through one (`pushdown_sorts_helper`). Both had the bug.
## What changes are included in this PR?
- Add an `input_fetch` helper in `sort_pushdown.rs` that returns `skip +
fetch` for
`GlobalLimitExec` and plain `fetch()` for every other operator.
- Use it at both sites where the pushed-down fetch is derived from the plan
node, so
the TopK below a `GlobalLimitExec` retains enough rows for the limit to
skip and
still return `fetch` rows.
## What is the testing strategy for this PR?
- `limit.slt`: new `EXPLAIN` + result test for `LIMIT 4 OFFSET 3` under
`ORDER BY`,
showing `TopK(fetch=7)` below `GlobalLimitExec: skip=3, fetch=4` and the
correct
4 rows.
- `ensure_requirements.rs`: two unit tests, one per code path
(`test_sort_pushed_below_limit_with_skip_keeps_skip_rows` and
`test_parent_ordering_over_limit_with_skip_keeps_skip_rows`), asserting the
pushed sort has `fetch=15` for `skip=5, fetch=10`.
## Are there any user-facing changes?
Queries with a sort above `LIMIT ... OFFSET` now return the correct number
of rows.
No API changes.
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