> ## Documentation index
> Fetch the complete documentation index at: https://upstash.com/docs/llms.txt
> Search it with GET https://upstash.com/docs/search?q=<query>.
> Use these to discover all available pages before exploring further.

# $rrf

> Merge the results of several retrievers with Reciprocal Rank Fusion.

The `$rrf` retriever runs several retrievers and merges their results with Reciprocal Rank Fusion
(RRF). Each retriever ranks its own results. A document gets points from every retriever that returns
it, so documents that rank high in several retrievers come first.

Use `$rrf` for hybrid search, to merge a full-text query with an [`$ann`](./ann) semantic search. It
also merges full-text queries only, or `$ann` retrievers only.

```json
{
  "$rrf": {
    "retrievers": [
      { "description": "noise cancelling headphones" },
      { "$ann": { "field": "description", "query": "noise cancelling headphones" } }
    ]
  }
}
```

### Parameters

| Parameter | Description | Default |
|---|---|---|
| `retrievers` | Required. 2 to 10 retrievers to merge. Each entry is a query object, a [`$standard`](./standard), or an [`$ann`](./ann). | |
| `rankConstant` | An integer between `1` and `2147483647`. Higher values give lower-ranked results more influence. | `60` |
| `window` | An integer between `1` and `11000`. The number of results each retriever contributes, and the maximum number of merged results. See [Paging](#paging). | `LIMIT` |

### Scoring

Each retriever that returns a document adds `weight / (rankConstant + rank)` to its score. `rank` is the
position of the document in that retriever's results, starting at `1`.

These scores are small numbers. Compare them only with other results of the same query.

### Weights

A weight scales how much a retriever adds to the score. All retrievers have a weight of `1` by default.

- To set the weight of a query object, wrap it in [`$standard`](./standard).
- An `$ann` retriever takes `weight` directly.
- A weight must be `0` or greater. With a weight of `0`, the documents of that retriever stay in the
  results, but the retriever adds nothing to their score.

### Paging

`window` sets how deep `$rrf` looks:

- Each retriever contributes up to `window` results, and the merged list is cut at `window`.
- When `window` is not set, it is equal to `LIMIT`, which is `10` by default.
- `LIMIT` cannot be larger than `window`.
- To use `OFFSET`, set `window`. A page that starts at or after `window` is empty.

Keep the same `window` on every page so that pages do not overlap.

### Examples

#### Hybrid Search

Merge a full-text query and a semantic search on the same field:

<Tabs>

<Tab title="Redis CLI">
```bash
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"description": "noise cancelling headphones"}, {"$ann": {"field": "description", "query": "noise cancelling headphones"}}]}}'
```
</Tab>

</Tabs>

#### Weighted Hybrid Search

Give the full-text query twice the weight of the semantic search:

<Tabs>

<Tab title="Redis CLI">
```bash
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"$standard": {"query": {"description": "noise cancelling headphones"}, "weight": 2}}, {"$ann": {"field": "description", "query": "noise cancelling headphones", "weight": 1}}]}}'
```
</Tab>

</Tabs>

#### Full-Text Queries Only

`$rrf` does not need `HYBRID` fields:

<Tabs>

<Tab title="Redis CLI">
```bash
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"name": "wireless headphones"}, {"description": "bluetooth"}]}}'
```
</Tab>

</Tabs>

#### Paging Through Results

Set `window` and keep it the same on every page:

<Tabs>

<Tab title="Redis CLI">
```bash
# Page 1
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"description": "headphones"}, {"$ann": {"field": "description", "query": "headphones"}}], "window": 100}}' LIMIT 10

# Page 2
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"description": "headphones"}, {"$ann": {"field": "description", "query": "headphones"}}], "window": 100}}' LIMIT 10 OFFSET 10
```
</Tab>

</Tabs>

See [Query Options](./overview#query-options) for the `SEARCH.QUERY` options that work with `$rrf`, and
[Counting and Aggregations](./overview#counting-and-aggregations) for `SEARCH.COUNT` and
`SEARCH.AGGREGATE`.