Similar to dense vector search, sparse vector search is an essential feature of TopK. With that in mind, it is designed to:
  • Provide 100% recall.
  • Support learned sparse vector representations, for example SPLADE.
  • Provide consistent low latency (<20ms p99). Check out our benchmarks.
  • Support large-scale single-collection as well as multi-tenant use cases.

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Prerequisites

Define a schema with a sparse vector field, either f32_sparse_vector() or u8_sparse_vector(), and add a vector_index() to it: 
from topk_sdk.schema import text, f32_sparse_vector, vector_index

client.collections().create(
    "books",
    schema={
        "title": text().required(),
        "title_embedding": f32_sparse_vector()
          .required()
          .index(vector_index(metric = "dot_product")),
    },
)
Sparse vectors do not have a fixed dimension, so you don’t need to specify the vector dimension when defining the field.
TopK only supports dot_product metric for sparse vectors which is compatible with both fixed and learned sparse vector representations.

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Find the most relevant documents

To find the top-k most relevant results for the query, use the vector_distance() function. This function computes a score between the provided sparse query and indexed sparse vectors which can then be used to sort the results. 
from topk_sdk.query import select, field, fn
from topk_sdk.data import f32_sparse_vector

docs = client.collection("books").query(
    select(
        "title",
        published_year=field("published_year"),
        # Compute relevance score between the sparse vector embedding of the string "epic fantasy adventure"
        # and the embedding stored in the `title_embedding` field.
        title_score=fn.vector_distance(
          "title_embedding",
          f32_sparse_vector({0: 0.12, 6: 0.67, ...}),
        )
    )
    # Return top 10 results
    .topk(field("title_score"), 10)
)

# Example results:
[
  {
    "_id": "2",
    "title": "Lord of the Rings",
    "title_score": 0.8150404095649719
  },
  {
    "_id": "1",
    "title": "The Catcher in the Rye",
    "title_score": 0.7825378179550171,
  }
]
Let’s break down the example above:
  1. Compute the sparse dot product between the query embedding and the title_embedding field using the vector_distance() function.
  2. Store the computed dot product score in the title_score field.
  3. Return the top 10 results sorted by the title_score field in a descending order.

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Combine sparse vector search with metadata filtering

Sparse vector search can be easily combined with metadata filtering by adding a filter() stage to the query: 
from topk_sdk.query import select, field, fn
from topk_sdk.data import f32_sparse_vector

docs = client.collection("books").query(
    select(
        "title",
        title_score=fn.vector_distance(
          "title_embedding",
          f32_sparse_vector({0: 0.12, 6: 0.67, ...}),
        )
        published_year=field("published_year"),
    )
    .filter(field("published_year") > 2000)
    .topk(field("title_score"), 10)
)