Drew Gallardo 41ac32a344 feat(rust): add blob read and materialization APIs (#3562)
This PR is for the Read path against blob v2. #3528 handles declare +
write, and this this adds materialization on local tables.

- blob_columns()
- fetch_blobs(column, row_ids) → bytes
- fetch_blob_files(column, row_ids) → lazy handles
- Pass _rowid from query().with_row_id(). Remote returns NotSupported.
(for now)

### Use cases

search, grab row ids, materialize images:

```rust
let row_ids = /* _rowid from hits */;
let images = table.fetch_blobs("image", &row_ids).await?;
```

Large blobs: open handles, read only what you need:

```rust
let handles = table.fetch_blob_files("image", &row_ids).await?;
let bytes = handles[0].as_ref().unwrap().read().await?;
```

Filter then batch fetch: collect ids from a filter, one call.
Multiple blob columns: image and thumbnail independently.
Row ids from before compact: still resolve.

### Alignment note
Lance `read_blobs` drops null rows. We descriptor-take first, read
non-null ids, re-expand to match input order. Null and zero-length blobs
come back null/None. Bytes path sets `preserve_order(true)`. So I added:

```
TODO(lance): expose selection_index or an aligned execute so we can drop the pre-read.
```

### Tests
`cargo test -p lancedb --test blob_integration`
- 30 tests covering nulls, reorder, dups, cross-fragment bytes + files,
compact, delete, legacy v1 errors.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-06-23 06:58:26 -07:00
2023-03-17 18:15:19 -07:00
2025-03-10 09:01:23 -07:00

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