> ## 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.

# Overview

> Retrievers run one or more queries and return the top documents.

A retriever runs one or more queries and returns the top documents. Queries describe which documents
match. Retrievers decide how those documents are found and ranked.

### Available Retrievers

| Retriever | Description |
|---|---|
| [`$rrf`](./rrf) | Merges the results of 2 to 10 retrievers with Reciprocal Rank Fusion. |
| [`$ann`](./ann) | Finds the documents whose `HYBRID` field is closest in meaning to a text. |
| [`$standard`](./standard) | Runs one query. Use it inside `$rrf` to set the weight of a query. |

<Tabs>

<Tab title="Redis CLI">
```bash
# Search a HYBRID field by meaning
SEARCH.QUERY products '{"$ann": {"field": "description", "query": "something to block noise on a flight"}}'

# Merge a full-text query and a semantic search
SEARCH.QUERY products '{"$rrf": {"retrievers": [{"description": "noise cancelling headphones"}, {"$ann": {"field": "description", "query": "noise cancelling headphones"}}]}}'
```
</Tab>

</Tabs>

### Where to Use Retrievers

A retriever must be the only key of its object. You can use it in two places:

- At the root of the query in [`SEARCH.QUERY`](/redis/commands/search/search-query),
  [`SEARCH.COUNT`](/redis/commands/search/search-count), and
  [`SEARCH.AGGREGATE`](/redis/commands/search/search-aggregate).
- As an entry of `$rrf` `retrievers`.

A retriever cannot be used inside a query, such as inside `$must`, `$should`, `$mustNot`, or an `$ann`
filter. `$rrf` cannot be used inside another `$rrf`.

### Query Options

Some `SEARCH.QUERY` options do not work with retrievers:

| Option | With `$rrf` | With `$ann` at the root |
|---|---|---|
| `ORDERBY` | Not supported | Not supported |
| `SCOREFUNC` | Supported. It changes the scores of the full-text queries before their results are merged. It does not change `$ann` results. | Not supported |
| `HIGHLIGHT` | Supported | Not supported |

`$standard` at the root supports all options, the same as a plain query.

### Counting and Aggregations

`SEARCH.COUNT` and `SEARCH.AGGREGATE` accept `$rrf` when it has no `$ann` retriever.
They use every document that matches any of the queries. `window` and `weight` do not apply, so the
count can be larger than the number of results `SEARCH.QUERY` returns.

`SEARCH.COUNT` and `SEARCH.AGGREGATE` do not support `$ann`.