Frequently Asked Questions

How many vectors can be stored in a single table?

A non-partitioned table has a limit of 32 TB by default in Postgres. A partitioned table can have thousands of partitions of that size.

Is replication supported?

Yes, pgvector uses the write-ahead log (WAL), which allows for replication and point-in-time recovery.

What if I want to index vectors with more than 2,000 dimensions?

You can use half-precision vectors or half-precision indexing to index up to 4,000 dimensions or binary quantization to index up to 64,000 dimensions. Other options are indexing subvectors (for models that support it) or dimensionality reduction.

Can I store vectors with different dimensions in the same column?

You can use vector as the type (instead of vector(n)).

CREATE TABLE embeddings (model_id bigint, item_id bigint, embedding vector, PRIMARY KEY (model_id, item_id));

However, you can only create indexes on rows with the same number of dimensions (using expression and partial indexing):

CREATE INDEX ON embeddings USING hnsw ((embedding::vector(3)) vector_l2_ops) WHERE (model_id = 123);

and query with:

SELECT * FROM embeddings WHERE model_id = 123 ORDER BY embedding::vector(3) <-> '[3,1,2]' LIMIT 5;

Can I store vectors with more precision?

You can use the double precision[] or numeric[] type to store vectors with more precision.

CREATE TABLE items (id bigserial PRIMARY KEY, embedding double precision[]);

-- use {} instead of [] for Postgres arrays
INSERT INTO items (embedding) VALUES ('{1,2,3}'), ('{4,5,6}');

Optionally, add a check constraint to ensure data can be converted to the vector type and has the expected dimensions.

ALTER TABLE items ADD CHECK (vector_dims(embedding::vector) = 3);

Use expression indexing to index (at a lower precision):

CREATE INDEX ON items USING hnsw ((embedding::vector(3)) vector_l2_ops);

and query with:

SELECT * FROM items ORDER BY embedding::vector(3) <-> '[3,1,2]' LIMIT 5;

Do indexes need to fit into memory?

No, but like other index types, you’ll likely see better performance if they do. You can get the size of an index with:

SELECT pg_size_pretty(pg_relation_size('index_name'));