sqlite-vec/ARCHITECTURE.md

# `sqlite-vec` Architecture

Internal documentation for how `sqlite-vec` works under-the-hood. Not meant for
users of the `sqlite-vec` project, consult
[the official `sqlite-vec` documentation](https://alexgarcia.xyz/sqlite-vec) for
how-to-guides. Rather, this is for people interested in how `sqlite-vec` works
and some guidelines to any future contributors.

Very much a WIP.

## `vec0`

### Shadow Tables

#### `xyz_chunks`

- `chunk_id INTEGER`
- `size INTEGER`
- `validity BLOB`
- `rowids BLOB`

#### `xyz_rowids`

- `rowid INTEGER`
- `id`
- `chunk_id INTEGER`
- `chunk_offset INTEGER`

#### `xyz_vector_chunksNN`

- `rowid INTEGER`
- `vector BLOB`

#### `xyz_auxiliary`

- `rowid INTEGER`
- `valueNN [type]`

#### `xyz_metadatachunksNN`

- `rowid INTEGER`
- `data BLOB`

#### `xyz_metadatatextNN`

- `rowid INTEGER`
- `data TEXT`

### idxStr

The `vec0` idxStr is a string composed of single "header" character and 0 or
more "blocks" of 4 characters each.

The "header" charcter denotes the type of query plan, as determined by the
`enum vec0_query_plan` values. The current possible values are:

| Name                       | Value | Description                                                            |
| -------------------------- | ----- | ---------------------------------------------------------------------- |
| `VEC0_QUERY_PLAN_FULLSCAN` | `'1'` | Perform a full-scan on all rows                                        |
| `VEC0_QUERY_PLAN_POINT`    | `'2'` | Perform a single-lookup point query for the provided rowid             |
| `VEC0_QUERY_PLAN_KNN`      | `'3'` | Perform a KNN-style query on the provided query vector and parameters. |

Each 4-character "block" is associated with a corresponding value in `argv[]`.
For example, the 1st block at byte offset `1-4` (inclusive) is the 1st block and
is associated with `argv[1]`. The 2nd block at byte offset `5-8` (inclusive) is
associated with `argv[2]` and so on. Each block describes what kind of value or
filter the given `argv[i]` value is.

#### `VEC0_IDXSTR_KIND_KNN_MATCH` (`'{'`)

`argv[i]` is the query vector of the KNN query.

The remaining 3 characters of the block are `_` fillers.

#### `VEC0_IDXSTR_KIND_KNN_K` (`'}'`)

`argv[i]` is the limit/k value of the KNN query.

The remaining 3 characters of the block are `_` fillers.

#### `VEC0_IDXSTR_KIND_KNN_ROWID_IN` (`'['`)

`argv[i]` is the optional `rowid in (...)` value, and must be handled with
[`sqlite3_vtab_in_first()` / `sqlite3_vtab_in_next()`](https://www.sqlite.org/c3ref/vtab_in_first.html).

The remaining 3 characters of the block are `_` fillers.

#### `VEC0_IDXSTR_KIND_KNN_PARTITON_CONSTRAINT` (`']'`)

`argv[i]` is a "constraint" on a specific partition key.

The second character of the block denotes which partition key to filter on,
using `A` to denote the first partition key column, `B` for the second, etc. It
is encoded with `'A' + partition_idx` and can be decoded with `c - 'A'`.

The third character of the block denotes which operator is used in the
constraint. It will be one of the values of `enum vec0_partition_operator`, as
only a subset of operations are supported on partition keys.

The fourth character of the block is a `_` filler.

#### `VEC0_IDXSTR_KIND_POINT_ID` (`'!'`)

`argv[i]` is the value of the rowid or id to match against for the point query.

The remaining 3 characters of the block are `_` fillers.

#### `VEC0_IDXSTR_KIND_METADATA_CONSTRAINT` (`'&'`)

`argv[i]` is the value of the `WHERE` constraint for a metdata column in a KNN
query.

The second character of the block denotes which metadata column the constraint
belongs to, using `A` to denote the first metadata column column, `B` for the
second, etc. It is encoded with `'A' + metadata_idx` and can be decoded with
`c - 'A'`.

The third character of the block is the constraint operator. It will be one of
`enum vec0_metadata_operator`, as only a subset of operators are supported on
metadata column KNN filters.

The foruth character of the block is a `_` filler.