Conduct a Vector Similarity Search

This topic describes how to search entities with Milvus.

A vector similarity search in Milvus calculates the distance between query vector(s) and vectors in the collection with specified similarity metrics, and returns the most similar results. You can perform a hybrid search by specifying a boolean expression that filters the scalar field or the primary key field.

The following example shows how to perform a vector similarity search on a 2000-row dataset of book ID (primary key), word count (scalar field), and book introduction (vector field), simulating the situation that you search for certain books based on their vectorized introductions. Milvus will return the most similar results according to the query vector and search parameters you have defined.

Load collection

All search and query operations within Milvus are executed in memory. Load the collection to memory before conducting a vector similarity search.

from pymilvus import Collection
collection = Collection("book")      # Get an existing collection.
collection.load()

await milvusClient.loadCollection({
  collection_name: "book",
});

err := milvusClient.LoadCollection(
  context.Background(),   // ctx
  "book",                 // CollectionName
  false                   // async
)
if err != nil {
  log.Fatal("failed to load collection:", err.Error())
}

milvusClient.loadCollection(
  LoadCollectionParam.newBuilder()
          .withCollectionName("book")
          .build()
);

load -c book

curl -X 'POST' \
  'http://localhost:9091/api/v1/collection/load' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
    "collection_name": "book"
  }'

Prepare search parameters

Prepare the parameters that suit your search scenario. The following example defines that the search will calculate the distance with Euclidean distance, and retrieve vectors from ten closest clusters built by the IVF_FLAT index.

search_params = {
    "metric_type": "L2", 
    "offset": 0, 
    "ignore_growing": False, 
    "params": {"nprobe": 10}
}

const searchParams = {
    params: { nprobe: 1024 }
};

sp, _ := entity.NewIndexIvfFlatSearchParam( // NewIndex*SearchParam func
    10,                                  // searchParam
)

opt := client.SearchQueryOptionFunc(func(option *client.SearchQueryOption) {
    option.Limit = 3
    option.Offset = 0
    option.ConsistencyLevel = entity.ClStrong
    option.IgnoreGrowing = false
})

final Integer SEARCH_K = 2;                       // TopK
final String SEARCH_PARAM = "{\"nprobe\":10, \"offset\":0}";    // Params

search

Collection name (book): book

The vectors of search data(the length of data is number of query (nq), the dim of every vector in data must be equal to vector field’s of collection. You can also import a csv file without headers): [[0.1, 0.2]]

The vector field used to search of collection (book_intro): book_intro

Metric type: L2

Search parameter nprobe's value: 10

The max number of returned record, also known as topk: 10

The boolean expression used to filter attribute []: 

The names of partitions to search (split by "," if multiple) ['_default'] []: 

timeout []:

Guarantee Timestamp(It instructs Milvus to see all operations performed before a provided timestamp. If no such timestamp is provided, then Milvus will search all operations performed to date) [0]: 

Travel Timestamp(Specify a timestamp in a search to get results based on a data view) [0]:

# Search entities based on a given vector.
curl --request POST \
     --url '${MILVUS_HOST}:${MILVUS_PORT}/v1/vector/search' \
     --header 'Authorization: Bearer <TOKEN>' \
     --header 'accept: application/json' \
     --header 'content-type: application/json'
     -d '{
        "collectionName": "collection1",
        "vector": [0.0128121, 0.029119, .... , 0.09121]
      }'

# Search entities and return specific fields.
curl --request POST \
     --url '${MILVUS_HOST}:${MILVUS_PORT}/v1/vector/search' \
     --header 'Authorization: Bearer <TOKEN>' \
     --header 'accept: application/json' \
     --header 'content-type: application/json'
     -d '{
       "collectionName": "collection1",
       "outputFields": ["id", "name", "feature", "distance"],
       "vector": [0.0128121, 0.029119, .... , 0.09121],
       "filter": "id in (1, 2, 3)",
       "limit": 100,
       "offset": 0
     }'

{
    "code": 200,
    "data": {}
}

Conduct a vector search

Search vectors with Milvus. To search in a specific partition, specify the list of partition names.

Milvus supports setting consistency level specifically for a search. The example in this topic sets the consistency level as Strong. You can also set the consistency level as Bounded, Session or Eventually. See Consistency for more information about the four consistency levels in Milvus.

results = collection.search(
    data=[[0.1, 0.2]], 
    anns_field="book_intro", 
    # the sum of `offset` in `param` and `limit` 
    # should be less than 16384.
    param=search_params,
    limit=10,
    expr=None,
    # set the names of the fields you want to 
    # retrieve from the search result.
    output_fields=['title'],
    consistency_level="Strong"
)

# get the IDs of all returned hits
results[0].ids

# get the distances to the query vector from all returned hits
results[0].distances

# get the value of an output field specified in the search request.
hit = results[0][0]
hit.entity.get('title')

const results = await milvusClient.search({
    collection_name: "book",
    vector: [0.1, 0.2],
    filter: null,
    // the sum of `limit` and `offset` should be less than 16384.
    limit: 10,
    offset: 2,
    metric_type: MetricType.L2,
    param: searchParams,
    consistency_level: ConsistencyLevelEnum.Strong,
});

searchResult, err := milvusClient.Search(
    context.Background(),                    // ctx
    "book",                                  // CollectionName
    []string{},                              // partitionNames
    "",                                      // expr
    []string{"book_id"},                     // outputFields
    []entity.Vector{entity.FloatVector([]float32{0.1, 0.2})}, // vectors
    "book_intro",                            // vectorField
    entity.L2,                               // metricType
    10,                                      // topK
    sp,                                      // sp
    opt,
)
if err != nil {
    log.Fatal("fail to search collection:", err.Error())
}

List<String> search_output_fields = Arrays.asList("book_id");
List<List<Float>> search_vectors = Arrays.asList(Arrays.asList(0.1f, 0.2f));

SearchParam searchParam = SearchParam.newBuilder()
        .withCollectionName("book")
        .withConsistencyLevel(ConsistencyLevelEnum.STRONG)
        .withMetricType(MetricType.L2)
        .withOutFields(search_output_fields)
        .withTopK(SEARCH_K)
        .withVectors(search_vectors)
        .withVectorFieldName("book_intro")
        .withParams(SEARCH_PARAM)
        .build();
R<SearchResults> respSearch = milvusClient.search(searchParam);

# Follow the previous step.

# Follow the previous step.

Check the primary key values of the most similar vectors and their distances.

results[0].ids
results[0].distances

console.log(results.results)

fmt.Printf("%#v\n", searchResult)
for _, sr := range searchResult {
    fmt.Println(sr.IDs)
    fmt.Println(sr.Scores)
}

SearchResultsWrapper wrapperSearch = new SearchResultsWrapper(respSearch.getData().getResults());
System.out.println(wrapperSearch.getIDScore(0));
System.out.println(wrapperSearch.getFieldData("book_id", 0));

# Milvus CLI automatically returns the primary key values of the most similar vectors and their distances.

Release the collection loaded in Milvus to reduce memory consumption when the search is completed.

collection.release()

await milvusClient.releaseCollection({  collection_name: "book",});

err := milvusClient.ReleaseCollection(
    context.Background(),                            // ctx
    "book",                                          // CollectionName
)
if err != nil {
    log.Fatal("failed to release collection:", err.Error())
}

milvusClient.releaseCollection(
        ReleaseCollectionParam.newBuilder()
                .withCollectionName("book")
                .build());

release -c book

curl -X 'DELETE' \
  'http://localhost:9091/api/v1/collection/load' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
    "collection_name": "book"
  }'

Limits

What's next

• Learn more basic operations of Milvus:

  • Query vectors
  • Conduct a hybrid search

• Explore API references for Milvus SDKs:

  • PyMilvus API reference
  • Node.js API reference
  • Go API reference
  • Java API reference