Search

Searching for the nearest vectors is at the core of many representational learning applications. Modern neural networks are trained to transform objects into vectors so that objects close in the real world appear close in vector space. It could be, for example, texts with similar meanings, visually similar pictures, or songs of the same genre.

Embeddings

Metrics

There are many ways to estimate the similarity of vectors with each other. In Qdrant terms, these ways are called metrics. The choice of metric depends on vectors obtaining and, in particular, on the method of neural network encoder training.

Qdrant supports these most popular types of metrics:

The most typical metric used in similarity learning models is the cosine metric.

Embeddings

Qdrant counts this metric in 2 steps, due to which a higher search speed is achieved. The first step is to normalize the vector when adding it to the collection. It happens only once for each vector.

The second step is the comparison of vectors. In this case, it becomes equivalent to dot production - a very fast operation due to SIMD.

Query planning

Depending on the filter used in the search - there are several possible scenarios for query execution. Qdrant chooses one of the query execution options depending on the available indexes, the complexity of the conditions and the cardinality of the filtering result. This process is called query planning.

The strategy selection process relies heavily on heuristics and can vary from release to release. However, the general principles are:

  • planning is performed for each segment independently (see storage for more information about segments)
  • prefer a full scan if the amount of points is below a threshold
  • estimate the cardinality of a filtered result before selecting a strategy
  • retrieve points using payload index (see indexing) if cardinality is below threshold
  • use filterable vector index if the cardinality is above a threshold

You can adjust the threshold using a configuration file, as well as independently for each collection.

Search API

Let’s look at an example of a search query.

REST API - API Schema definition is available here

POST /collections/{collection_name}/points/search

{
    "filter": {
        "must": [
            {
                "key": "city",
                "match": {
                    "value": "London"
                }
            }
        ]
    },
    "params": {
        "hnsw_ef": 128
    },
    "vector": [0.2, 0.1, 0.9, 0.7],
    "limit": 3
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

client.search(
    collection_name="{collection_name}",
    query_filter=models.Filter(
        must=[
            models.FieldCondition(
                key="city",
                match=models.MatchValue(
                    value="London",
                ),
            )
        ]
    ),
    search_params=models.SearchParams(
        hnsw_ef=128
    ),
    query_vector=[0.2, 0.1, 0.9, 0.7],
    limit=3,
)

In this example, we are looking for vectors similar to vector [0.2, 0.1, 0.9, 0.7]. Parameter limit (or its alias - top) specifies the amount of most similar results we would like to retrieve.

Values under the key params specify custom parameters for the search. Currently, it could be:

  • hnsw_ef - value that specifies ef parameter of the HNSW algorithm.

Since the filter parameter is specified, the search is performed only among those points that satisfy the filter condition. See details of possible filters and their work in the filtering section.

Example result of this API would be

{
  "result": [
    { "id": 10, "score": 0.81 },
    { "id": 14, "score": 0.75 },
    { "id": 11, "score": 0.73 }
  ],
  "status": "ok",
  "time": 0.001
}

The result contains ordered by score list of found point ids.

Available since v0.10.0

If the collection was created with multiple vectors, the name of the vector to use for searching should be provided:

POST /collections/{collection_name}/points/search

{
    "vector": {
        "name": "image",
        "vector": [0.2, 0.1, 0.9, 0.7]
    },
    "limit": 3
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

client.search(
    collection_name="{collection_name}",
    query_vector=("image", [0.2, 0.1, 0.9, 0.7]),
    limit=3,
)

Search is processing only among vectors with the same name.

Filtering results by score

In addition to payload filtering, it might be useful to filter out results with a low similarity score. For example, if you know the minimal acceptance score for your model and do not want any results which are less similar than the threshold. In this case, you can use score_threshold parameter of the search query. It will exclude all results with a score worse than the given.

Payload and vector in the result

By default, retrieval methods do not return any stored information. Additional parameters with_vector and with_payload could alter this behavior.

Example:

POST /collections/{collection_name}/points/search

{
    "vector": [0.2, 0.1, 0.9, 0.7],
    "with_vector": true,
    "with_payload": true
}

client.search(
    collection_name="{collection_name}",
    query_vector=[0.2, 0.1, 0.9, 0.7],
    with_vector=True,
    with_payload=True,
)

Parameter with_payload might also be used to include or exclude specific fields only:

POST /collections/{collection_name}/points/search

{
    "vector": [0.2, 0.1, 0.9, 0.7],
    "with_payload": {
      "exclude": ["city"]
    }
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

client.search(
    collection_name="{collection_name}",
    query_vector=[0.2, 0.1, 0.9, 0.7],
    with_payload=models.PayloadSelectorExclude(
        exclude=["city"],
    ),
)

Batch search API

Available since v0.10.0

The batch search API enables to perform multiple search requests via a single request.

Its semantic is straightforward, n batched search requests are equivalent to n singular search requests.

This approach has several advantages. Logically, fewer network connections are required which can be very beneficial on its own.

More importantly, batched requests will be efficiently processed via the query planner which can detect and optimize requests if they have the same filter.

This can have a great effect on latency for non trivial filters as the intermediary results can be shared among the request.

In order to use it, simply pack together your search requests. All the regular attributes of a search request are of course available.

POST /collections/{collection_name}/points/search/batch

{
    "searches": [
        {
            "filter": {
                "must": [
                    {
                        "key": "city",
                        "match": {
                            "value": "London"
                        }
                    }
                ]
            },
            "vector": [0.2, 0.1, 0.9, 0.7],
            "limit": 3
        },
        {
            "filter": {
                "must": [
                    {
                        "key": "city",
                        "match": {
                            "value": "London"
                        }
                    }
                ]
            },
            "vector": [0.5, 0.3, 0.2, 0.3],
            "limit": 3
        }
    ]
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

filter = models.Filter(
    must=[
        models.FieldCondition(
            key="city",
            match=models.MatchValue(
                value="London",
            ),
        )
    ]
)

search_queries = [
    SearchRequest(
        vector=[0.2, 0.1, 0.9, 0.7],
        filter=filter,
        limit=3
    ),
    SearchRequest(
        vector=[0.5, 0.3, 0.2, 0.3],
        filter=filter,
        limit=3
    )
]

client.search_batch(
    collection_name="{collection_name}",
    requests=search_queries
)

The result of this API contains one array per search requests.

{
  "result": [
    [
        { "id": 10, "score": 0.81 },
        { "id": 14, "score": 0.75 },
        { "id": 11, "score": 0.73 }
    ],
    [
        { "id": 1, "score": 0.92 },
        { "id": 3, "score": 0.89 },
        { "id": 9, "score": 0.75 }
    ]
  ],
  "status": "ok",
  "time": 0.001
}

Recommendation API

In addition to the regular search, Qdrant also allows you to search based on multiple vectors already stored in the collection. This API uses vector search without involving the neural network encoder for already encoded objects.

The recommendation API allows specifying several positive and negative vector IDs, which the service will combine into a certain average vector.

average_vector = avg(positive_vectors) + ( avg(positive_vectors) - avg(negative_vectors) )

If there is only one positive ID provided - this request is equivalent to the regular search with vector of that point.

Vector components that have a greater value in a negative vector are penalized, and those that have a greater value in a positive vector, on the contrary, are amplified. This average vector will be used to find the most similar vectors in the collection.

REST API - API Schema definition is available here

POST /collections/{collection_name}/points/recommend

{
  "filter": {
        "must": [
            {
                "key": "city",
                "match": {
                    "value": "London"
                }
            }
        ]
  },
  "negative": [718],
  "positive": [100, 231],
  "limit": 10
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

client.recommend(
    collection_name="{collection_name}",
    query_filter=models.Filter(
        must=[
            models.FieldCondition(
                key="city",
                match=models.MatchValue(
                    value="London",
                ),
            )
        ]
    ),
    negative=[718],
    positive=[100, 231],
    limit=10,
)

Example result of this API would be

{
  "result": [
    { "id": 10, "score": 0.81 },
    { "id": 14, "score": 0.75 },
    { "id": 11, "score": 0.73 }
  ],
  "status": "ok",
  "time": 0.001
}

Available since v0.10.0

If the collection was created with multiple vectors, the name of the vector should be specified in the recommendation request:

POST /collections/{collection_name}/points/recommend

{
  "positive": [100, 231],
  "negative": [718],
  "using": "image",
  "limit": 10
 }

client.recommend(
    collection_name="{collection_name}",
    positive=[100, 231],
    negative=[718],
    using="image",
    limit=10,
)

Parameter using specifies which stored vectors to use for the recommendation.

Batch recommendation API

Available since v0.10.0

Similar to the batch search API in terms of usage and advantages, it enables the batching of recommendation requests.

POST /collections/{collection_name}/points/recommend/batch

{
    "searches": [
        {
            "filter": {
                    "must": [
                        {
                            "key": "city",
                            "match": {
                                "value": "London"
                            }
                        }
                    ]
            },
            "negative": [718],
            "positive": [100, 231],
            "limit": 10
        },
        {
            "filter": {
                "must": [
                    {
                        "key": "city",
                        "match": {
                            "value": "London"
                        }
                    }
                    ]
            },
            "negative": [300],
            "positive": [200, 67],
            "limit": 10
        }
    ]
}

from qdrant_client import QdrantClient
from qdrant_client.http import models

client = QdrantClient(host="localhost", port=6333)

filter = models.Filter(
    must=[
        models.FieldCondition(
            key="city",
            match=models.MatchValue(
                value="London",
            ),
        )
    ]
)

recommend_queries = [
    models.RecommendRequest(
        positive=[100, 231],
        negative=[718],
        filter=filter,
        limit=3
    ),
    models.RecommendRequest(
        positive=[200, 67],
        negative=[300],
        filter=filter,
        limit=3
    )
]

client.recommend_batch(
    collection_name="{collection_name}",
    requests=recommend_queries
)

The result of this API contains one array per recommendation requests.

{
  "result": [
    [
        { "id": 10, "score": 0.81 },
        { "id": 14, "score": 0.75 },
        { "id": 11, "score": 0.73 }
    ],
    [
        { "id": 1, "score": 0.92 },
        { "id": 3, "score": 0.89 },
        { "id": 9, "score": 0.75 }
    ]
  ],
  "status": "ok",
  "time": 0.001
}

Pagination

Available since v0.8.3

Search and recommendation APIs allow to skip first results of the search and return only the result starting from some specified offset:

Example:

POST /collections/{collection_name}/points/search

{
    "vector": [0.2, 0.1, 0.9, 0.7],
    "with_vector": true,
    "with_payload": true,
    "limit": 10,
    "offset": 100
}

from qdrant_client import QdrantClient

client = QdrantClient(host="localhost", port=6333)

client.search(
    collection_name="{collection_name}",
    query_vector=[0.2, 0.1, 0.9, 0.7],
    with_vector=True,
    with_payload=True,
    limit=10,
    offset=100
)

Is equivalent to retrieving the 11th page with 10 records per page.

Vector-based retrieval in general and HNSW index in particular, are not designed to be paginated. It is impossible to retrieve Nth closest vector without retrieving the first N vectors first.

However, using the offset parameter saves the resources by reducing network traffic and the number of times the storage is accessed.

Using an offset parameter, will require to internally retrieve offset + limit points, but only access payload and vector from the storage those points which are going to be actually returned.