Add blog: Sort Pushdown in DataFusion: Skip Sorts, Skip I/O

[zhuqi-lucas]

Summary

A walkthrough of the sort pushdown work landed and in flight on Apache DataFusion. Opening as a draft to share the narrative early — the in-flight PRs the post discusses are still in flight, but the structure and the merged work (Phase 1 #19064, Phase 2 #21182, BufferExec #21426, row-group reverse #18817) are in their final shape.

What this post covers

• Why SortExec is expensive, and what Exact / Inexact mean at runtime (static fetch vs TopK dynamic filter).
• Phase 1 (#19064) — the PushdownSort rule + reverse row-group case.
• Phase 2 (#21182) — statistics-based file sort that upgrades Unsupported to Exact, eliminating the SortExec on non-overlapping ASC scans. Includes the BufferExec compensation (#21426) so the SPM above doesn't lose its implicit memory buffer.
• Reverse scans — today's row-group reverse (#18817, Inexact) and the community decision to wait for arrow-rs page-level reverse (apache/arrow-rs#9937) before pursuing Exact reverse, after memory-profile pushback on the original row-group-level proposal.
• Benchmarks — 2.1×–49× on the ASC-LIMIT sort_pushdown suite.
• What's next — the dynamic / TopK-driven path (#21351 merged, #21733, #21712, #21956, #21580) including the precise RG-level pruning vs mid-stream early return distinction, and the EnsureRequirements unification (#21976).
• Links into the prior dynamic filters and limit pruning posts so the series reads as a coherent thread.

Why a draft

A few of the in-flight PRs the "What's next" section references may evolve in review (e.g. #21580 may be split into smaller pieces, dynamic RG scheduling on top of #21351 is described but not yet on a PR). Opening as draft so we can adjust wording as those land or change shape — happy to flip to ready for review when the dust settles, or earlier if reviewers prefer.

Test plan

• Rendered locally with the Pelican Docker image from the project README — images, internal links, code blocks, and tables all render correctly.
• All issue / PR / blog-post links checked against current state.