Index and query vectors

Learn how to index and query vector embeddings with Redis

Redis Query Engine lets you index vector fields in hash or JSON objects (see the Vectors reference page for more information). Among other things, vector fields can store text embeddings, which are AI-generated vector representations of the semantic information in pieces of text. The vector distance between two embeddings indicates how similar they are semantically. By comparing the similarity of an embedding generated from some query text with embeddings stored in hash or JSON fields, Redis can retrieve documents that closely match the query in terms of their meaning.

In the example below, we use the sentence-transformers library to generate vector embeddings to store and index with Redis Query Engine.

Initialize

Install redis-py if you have not already done so. Also, install sentence-transformers with the following command:

pip install sentence-transformers

In a new Python source file, start by importing the required classes:

from sentence_transformers import SentenceTransformer
from redis.commands.search.query import Query
from redis.commands.search.field import TextField, TagField, VectorField
from redis.commands.search.indexDefinition import IndexDefinition, IndexType

import numpy as np
import redis

The first of these imports is the SentenceTransformer class, which generates an embedding from a section of text. Here, we create an instance of SentenceTransformer that uses the all-MiniLM-L6-v2 model for the embeddings. This model generates vectors with 384 dimensions, regardless of the length of the input text, but note that the input is truncated to 256 tokens (see Word piece tokenization at the Hugging Face docs to learn more about the way tokens are related to the original text).

model = SentenceTransformer("sentence-transformers/all-MiniLM-L6-v2")

Create the index

Connect to Redis and delete any index previously created with the name vector_idx. (The dropindex() call throws an exception if the index doesn't already exist, which is why you need the try: except: block.)

r = redis.Redis(decode_responses=True)

try:
    r.ft("vector_idx").dropindex(True)
except redis.exceptions.ResponseError:
    pass

Next, we create the index. The schema in the example below specifies hash objects for storage and includes three fields: the text content to index, a tag field to represent the "genre" of the text, and the embedding vector generated from the original text content. The embedding field specifies HNSW indexing, the L2 vector distance metric, Float32 values to represent the vector's components, and 384 dimensions, as required by the all-MiniLM-L6-v2 embedding model.

schema = (
    TextField("content"),
    TagField("genre"),
    VectorField("embedding", "HNSW", {
        "TYPE": "FLOAT32",
        "DIM": 384,
        "DISTANCE_METRIC":"L2"
    })
)

r.ft("vector_idx").create_index(
    schema,
    definition=IndexDefinition(
        prefix=["doc:"], index_type=IndexType.HASH
    )
)

Add data

You can now supply the data objects, which will be indexed automatically when you add them with hset(), as long as you use the doc: prefix specified in the index definition.

Use the model.encode() method of SentenceTransformer as shown below to create the embedding that represents the content field. The astype() option that follows the model.encode() call specifies that we want a vector of float32 values. The tobytes() option encodes the vector components together as a single binary string rather than the default Python list of float values. Use the binary string representation when you are indexing hash objects (as we are here), but use the default list of float for JSON objects.

content = "That is a very happy person"

r.hset("doc:0", mapping={
    "content": content,
    "genre": "persons",
    "embedding": model.encode(content).astype(np.float32).tobytes(),
})

content = "That is a happy dog"

r.hset("doc:1", mapping={
    "content": content,
    "genre": "pets",
    "embedding": model.encode(content).astype(np.float32).tobytes(),
})

content = "Today is a sunny day"

r.hset("doc:2", mapping={
    "content": content,
    "genre": "weather",
    "embedding": model.encode(content).astype(np.float32).tobytes(),
})

Run a query

After you have created the index and added the data, you are ready to run a query. To do this, you must create another embedding vector from your chosen query text. Redis calculates the similarity between the query vector and each embedding vector in the index as it runs the query. It then ranks the results in order of this numeric similarity value.

The code below creates the query embedding using model.encode(), as with the indexing, and passes it as a parameter when the query executes (see Vector search for more information about using query parameters with embeddings).

q = Query(
    "*=>[KNN 3 @embedding $vec AS vector_distance]"
).return_field("score").dialect(2)

query_text = "That is a happy person"

res = r.ft("vector_idx").search(
    q, query_params={
        "vec": model.encode(query_text).astype(np.float32).tobytes()
    }
)

print(res)

The code is now ready to run, but note that it may take a while to complete when you run it for the first time (which happens because RedisVL must download the all-MiniLM-L6-v2 model data before it can generate the embeddings). When you run the code, it outputs the following result object (slightly formatted here for clarity):

Result{
    3 total,
    docs: [
        Document {
            'id': 'doc:0',
            'payload': None,
            'vector_distance': '0.114169985056',
            'content': 'That is a very happy person'
        },
        Document {
            'id': 'doc:1',
            'payload': None,
            'vector_distance': '0.610845386982',
            'content': 'That is a happy dog'
        },
        Document {
            'id': 'doc:2',
            'payload': None,
            'vector_distance': '1.48624813557',
            'content': 'Today is a sunny day'
        }
    ]
}

Note that the results are ordered according to the value of the vector_distance field, with the lowest distance indicating the greatest similarity to the query. As you would expect, the result for doc:0 with the content text "That is a very happy person" is the result that is most similar in meaning to the query text "That is a happy person".

Learn more

See Vector search for more information about the indexing options, distance metrics, and query format for vectors.