From 976edeb2ff3b936e864eaf02cb54553f00ad3242 Mon Sep 17 00:00:00 2001
From: Yang Cen <bubble-cal@outlook.com>
Date: Wed, 17 Jun 2026 19:28:42 +0800
Subject: [PATCH] feat(query): add approx mode to vector queries (#3549)

## Feature

### What is the new feature?

Adds Rust core API support for configuring vector query approximation
mode with `ApproxMode::{Fast, Normal, Accurate}`.

### Why do we need this feature?

Lance already exposes `lance_index::vector::ApproxMode` and scanner
support for controlling the speed/accuracy tradeoff for approximate
vector search. LanceDB Rust queries need to expose and pass this setting
through for local/native and remote vector searches.

### How does it work?

- Adds public `ApproxMode` in `rust/lancedb`, with lowercase serde,
`Default::Normal`, parse/display, and conversions to/from Lance's
`ApproxMode`.
- Adds `approx_mode: Option<ApproxMode>` to `VectorQueryRequest` and a
`VectorQuery::approx_mode(...)` builder.
- Applies the mode to native/local Lance scanners after `nearest(...)`
when explicitly set.
- Sends `approx_mode` in remote query JSON only when explicitly set;
default requests omit it.

## Validation

- `cargo fmt --all`
- `cargo test --quiet --features remote approx_mode`
- `cargo test --quiet --features remote
test_query_vector_default_values`
- `cargo check --quiet --features remote --tests --examples`
- `git diff --check`