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 indexing to index up to 4,000 dimensions or binary quantization to index up to 64,000 dimensions. Another option is dimensionality reduction.

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

You can use vector as the type (instead of vector(3)).

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'));