Create indexes and query vectors

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This document shows you how to use stored embeddings to generate indexes and query embeddings. For more information about storing embedding, see Store vector embeddings.

You can create ScaNN, IVF, IVFFlat, and HNSW indexes with AlloyDB.

Before you begin

Before you can start creating indexes, you must complete the following prerequisites.

  • Embedding vectors are added to a table in your AlloyDB database.

  • The vector extension version 0.5.0 or later that is based on pgvector, extended by Google for AlloyDB is installed.

    CREATE EXTENSION IF NOT EXISTS vector;

  • To generate ScaNN indexes, install the alloydb_scann extension in addition to the vector extension.

    CREATE EXTENSION IF NOT EXISTS alloydb_scann;

Create an index

You can create one of the following index types for tables in your database.

Create a ScaNN index

AlloyDB alloydb_scann, a

PostgreSQL extension developed by Google that implements a highly

efficient nearest-neighbor index powered by [the ScaNN

algorithm](https://github.com/google-research/google-research/blob/master/scann/docs/algorithms.md).

The ScaNN index is a tree-based quantization index for approximate

nearest neighbor search. It provides lower index building time and smaller

memory footprint as compared to HNSW. In addition, it provides faster QPS in

comparison to HNSW based on the workload.

a table in your AlloyDB database. If you try to generate a ScaNN index

on an empty or partitioned table, then you might encounter some issues. For more

information about the errors generated, see Troubleshoot ScaNN index errors.

Two-level tree ScaNN index

To apply a two-level tree index using the ScaNN algorithm to a column

containing stored vector embeddings, run the following DDL query:


CREATE INDEX INDEX_NAME ON TABLE

  USING scann (EMBEDDING_COLUMN DISTANCE_FUNCTION)

  WITH (num_leaves=NUM_LEAVES_VALUE);

Replace the following:

  • INDEX_NAME: the name of the index you want to

    create—for example, my-scann-index. The index names are shared

    across your database. Ensure that each index name is unique to each

    table in your database.

  • TABLE: the table to add the index to.

  • EMBEDDING_COLUMN: a column that stores vector

    data.

  • DISTANCE_FUNCTION: the distance function to use

    with this index. Choose one of the following:

    • L2 distance: l2

    • Dot product: dot_product

    • Cosine distance: cosine

  • NUM_LEAVES_VALUE: the number of partitions to apply to

    this index. Set to any value between 1 to 1048576. For more information

    about how to decide this value, see Tune a ScaNN index.

Three-level tree ScaNN index

To create a three-level tree index using the ScaNN algorithm to a column

containing stored vector embeddings, run the following DDL query:


CREATE INDEX INDEX_NAME ON TABLE

  USING scann (EMBEDDING_COLUMN DISTANCE_FUNCTION)

  WITH (num_leaves=NUM_LEAVES_VALUE, max_num_levels = MAX_NUM_LEVELS);

Replace the following:

  • MAX_NUM_LEVELS: the maximum number of levels of the

    K-means clustering tree. Set to 1(default) for two-level tree-based

    quantization and to 2 for three-level tree-based quantization.

After you create the index, you can run nearest-neighbor search queries that

make use of the index by following the instructions in [Make a nearest-neighbor

query with given text](#query).

The index parameters must be set to strike a right balance between QPS and

recall. For more information about tuning the ScaNN index, see [Tune a ScaNN

index](/alloydb/omni/containers/15.7.0/docs/ai/tune-indexes).

To create this index on an embedding column that uses the real[] data type

instead of vector, cast the column into the vector data type:


CREATE INDEX INDEX_NAME ON TABLE

  USING scann (CAST(EMBEDDING_COLUMN AS vector(DIMENSIONS)) DISTANCE_FUNCTION)

  WITH (num_leaves=NUM_LEAVES_VALUE, max_num_levels = MAX_NUM_LEVELS);

Replace DIMENSIONS with the dimensional width of the

embedding column. For more information about how to find the dimensions,

see the vector_dims function in [Vector

functions](https://github.com/pgvector/pgvector?tab=readme-ov-file#vector-functions).

To view the indexing progress, use the pg_stat_progress_create_index view:


SELECT * FROM pg_stat_progress_create_index;

The phase column shows the current state of your index creation, and the

building index: tree training phase disappears after the index is created.

To tune your index for a target recall and QPS balance, see Tune a ScaNN index.

Analyze your indexed table

After you create the ScaNN index, run the ANALYZE command to update statistics about your data.


ANALYZE TABLE;

Run a query

After you have stored and indexed embeddings in your database, you can start

querying using the [pgvector query

functionality](https://github.com/pgvector/pgvector#querying). You cannot run

bulk search queries using the alloydb_scann extension.

To find the nearest semantic neighbors for an embedding vector, you can run the

following example query, where you set the same distance function that you used

during the index creation.


  SELECT * FROM TABLE

    ORDER BY EMBEDDING_COLUMN DISTANCE_FUNCTION_QUERY ['EMBEDDING']

    LIMIT ROW_COUNT

Replace the following:

  • TABLE: the table containing the embedding to compare the

    text to.

  • INDEX_NAME: the name of the index you want to use—for

    example, my-scann-index.

  • EMBEDDING_COLUMN: the column containing the stored

    embeddings.

  • DISTANCE_FUNCTION_QUERY: the distance function to use with this

    query. Choose one of the following based on the distance function used

    while creating the index:

    • L2 distance: <->

    • Inner product: <#>

    • Cosine distance: <=>

  • EMBEDDING: the embedding vector you want to find the nearest stored

    semantic neighbors of.

  • ROW_COUNT: the number of rows to return.

    Specify 1 if you want only the single best match.

For more information about other query examples, see

Querying.

You can use also use the embedding() function to translate the

text into a vector. You apply the vector to one of the

pgvector nearest-neighbor operator, <-> for L2 distance, to find the database rows with the

most semantically similar embeddings.

Because embedding() returns a real array, you must explicitly cast the

embedding() call to vector in order to use these values with pgvector

operators.


  CREATE EXTENSION IF NOT EXISTS google_ml_integration;

  CREATE EXTENSION IF NOT EXISTS vector;



  SELECT * FROM TABLE

    ORDER BY EMBEDDING_COLUMN::vector

    <-> embedding('MODEL_IDVERSION_TAG', 'TEXT')

    LIMIT ROW_COUNT

Replace the following:

  • MODEL_ID: the ID of the model to query.

    If you are using the Vertex AI Model Garden, then specify text-embedding-005 as the model ID. These are the cloud-based models that AlloyDB can use for text embeddings. For more information, see Text embeddings.

  • Optional: VERSION_TAG: the version tag of the model to query. Prepend the tag with @.

    If you are using one of the text-embedding English models with Vertex AI, then specify one of the version tags—for example, text-embedding-005, listed in Model versions.

    Google strongly recommends that you always specify the version tag. If you don't specify the version tag, then AlloyDB always uses the latest model version, which might lead to unexpected results.

  • TEXT: the text to translate into a vector embedding.

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