[query api] don't propagate offset into prefetches #6357

coszio · 2025-04-09T23:10:39Z

One overlooked quirk when developing prefetches is that we should not propagate the main query offset into them.

In general terms, offsets are not compatible with prefetches. Let's look at one example:

Imagine we have a vector search prefetch (limit=5), and an order_by main query (limit=3). For simplicity, let's assume letters are point IDs, and numbers are the order_by values

with offset 0
prefetch candidates: [a: 5, b: 8, c: 3, d: 9, e: 2]

main query ordering: [e: 2, c: 3, a: 5, b: 8, d: 9]
main query results:  [e: 2, c: 3, a: 5]

----------------------------------------
with offset 3 (propagated)
prefetch candidates: [a: 5, b: 8, c: 3, d: 9, e: 2, f: 6, g: 1, h: 4]

main query ordering: [g: 1, e: 2, c: 3, h: 4, a: 5, f: 6, b: 8, d: 9]
main query results:  [h: 4, a: 5, f: 6]

As you can see, the second query, with an offset is not consistent with the first one. Result g will never appear.

With this PR, offset is not propagated anymore, so the limit in the prefetch is the total amount of offsetting that can happen for the request. If the user keeps paginating with offset and a fixed limit, results will be stable, but will eventually lead to an empty result.

coderabbitai · 2025-04-09T23:12:43Z

📝 Walkthrough

Walkthrough

The changes update the query handling and memory management modules. In the query processing logic, the adjustment of the limit by adding the offset has been removed from the main query code when prefetches are involved. A comment now clarifies that the limit should only be increased by the offset in specific cases without prefetches. Additionally, the recursive prefetch function has been modified by removing the redundant parameter used to pass offset adjustments, simplifying its signature and internal logic. In the memory management module, an unused import for handling file-related operations has been removed and re-scoped to be used only within the clear_disk_cache function. In the testing module, assertions and comments regarding the offset parameter and prefetch functionality have been updated for clarity. These changes streamline the code by reducing unnecessary computations and dependencies while maintaining its overall structure and functionality.

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lib/collection/src/operations/universal_query/planned_query.rs (5 hunks)

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lib/collection/src/operations/universal_query/planned_query.rs

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agourlay · 2025-04-10T08:09:56Z

lib/common/memory/src/madvise.rs

@@ -1,7 +1,6 @@
 //! Platform-independent abstractions over [`memmap2::Mmap::advise`]/[`memmap2::MmapMut::advise`]
 //! and [`memmap2::Advice`].

-use std::fs::File;


maybe unnecessary on Mac but breaks the build for everyone else :)

It was indeed 'unused' on macOS because it's only usage is gated in

#[cfg(any( target_os = "linux", target_os = "freebsd", target_os = "android", target_os = "fuchsia", target_os = "emscripten", target_os = "wasi", target_env = "uclibc", ))]

I've moved the import into that scope to resolve the problem.

72cbf76

timvisee

Could you add a test that asserts this behavior?

Prefetch with and without an offset, and assert that we eventually run out of results. That'll ensure the behavior we document actually happens in practice.

coszio · 2025-04-10T16:27:58Z

@timvisee There was actually a failing test (c5fbbad) that I have now edited to assert the new behavior.

agourlay · 2025-04-11T12:27:45Z

This may break the local mode congruence tests 🔮

* don't propagate offset into prefetches * I want to see CI with that change * Move import into platform specific scope * edit test to make sure offset is not propagated * fix planned query test --------- Co-authored-by: Arnaud Gourlay <arnaud.gourlay@gmail.com> Co-authored-by: timvisee <tim@visee.me>

Asserts fixed behavior in <#6412>, which broke before in <#6357>.

* Add test for limit and offset in query with prefetch Asserts fixed behavior in <#6412>, which broke before in <#6357>. * Fix typo Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> --------- Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

don't propagate offset into prefetches

c74adce

coszio requested a review from generall April 9, 2025 23:10

agourlay reviewed Apr 10, 2025

View reviewed changes

I want to see CI with that change

3f54a60

timvisee reviewed Apr 10, 2025

View reviewed changes

timvisee and others added 2 commits April 10, 2025 11:04

Move import into platform specific scope

72cbf76

edit test to make sure offset is not propagated

c5fbbad

fix planned query test

58c26fa

github-actions bot mentioned this pull request Apr 10, 2025

Flaky test hnsw_discover_test::hnsw_discover_precision #2973

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agourlay approved these changes Apr 11, 2025

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timvisee approved these changes Apr 11, 2025

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timvisee merged commit 634e506 into dev Apr 11, 2025
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timvisee deleted the don't-propagate-offset-into-prefetches branch April 11, 2025 12:11

coszio mentioned this pull request Apr 11, 2025

Specify that offset is not propagated into prefetches qdrant/landing_page#1564

Merged

coderabbitai bot mentioned this pull request Apr 22, 2025

Also use limit+offset with prefetches, we cut offset at collection level #6412

Merged

9 tasks

timvisee added a commit that referenced this pull request Apr 23, 2025

Add test for limit and offset in query with prefetch

b781198

Asserts fixed behavior in <#6412>, which broke before in <#6357>.

timvisee mentioned this pull request Apr 23, 2025

Add test for limit and offset in query with prefetch #6419

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