Search

Get the nearest 5 neighbors to a vector

SET vectors.hnsw_ef_search = 64;
SELECT * FROM items ORDER BY embedding <-> '[3,2,1]' LIMIT 5;

Operators

| Name | Description | | ---- ---------------------------- | | <-> | squared Euclidean distance | | <#> | negative dot product | | <=> | cosine distance |

For operator formula, see overview.

Filter

For a given category, get the nearest 10 neighbors to a vector

SELECT 1 FROM items WHERE category_id = 1 ORDER BY embedding <#> '[0.5,0.5,0.5]' limit 10

Query options

Search options are specified by PostgreSQL GUC.

Set ivf scan lists to 1 in session:

SET vectors.ivf_nprobe=1;

Set hnsw search scope to 40 in transaction:

SET LOCAL vectors.hnsw_ef_search=40;

Set search mode to vbase as system default:

ALTER SYSTEM SET vectors.search_mode=vbase;

Query options for ivf:

Option	Type	Range	Default	Description
vectors.ivf_nprobe	integer	[1, 1000000]	10	Number of lists to scan.

Query options for hnsw:

Option	Type	Range	Default	Description
vectors.hnsw_ef_search	integer	[1, 65535]	100	Search scope of HNSW.

Query options for general:

Option	Type	Range	Default	Description
vectors.enable_index	boolean		on	Enables or disables the query planner.
vectors.search_mode	enum	"basic", "vbase"	"basic"	Search mode.
vectors.enable_prefilter	boolean		on	Enables or disables the prefilter.

Advanced usage

Sometimes you expect the search to return the exact number of vectors equal to LIMIT, but it can't:

SELECT COUNT(1) FROM (SELECT 1 FROM t WHERE (category_id = 1) ORDER BY val <-> '[1,1,1]' limit 10) t2;
--- returns 1, much less than 10

That is why we introduce search mode and prefilter.

Search modes

There are two search modes: basic and vbase.

basic

basic is the default search mode.

In this mode, the filter is applied after vectors.hnsw_ef_search vectors are returned. Therefore you need to increase vectors.hnsw_ef_search until filtered vectors are sufficient.

The appropriate value depends on the input data distribution and filtering rate. Too large vectors.hnsw_ef_search will result in wasted memory.

It's recommended in these situations:

• Search without filter and transaction
• Returning insufficient vectors is acceptable
• The `vbase' search mode fails due to out-of-memory

vbase

vbase is the recommended search mode when any filter is enabled.

In this mode, the filter is applied after range vectors are returned. The value of range is automatically chosen by the vbase algorithm. It is transparent to the user.

In most cases, vbase mode would return enough vectors for your filter. For how it works, see the thesis VBASE: Unifying Online Vector Similarity Search and Relational Queries via Relaxed Monotonicity.

It's recommended in these situations:

• Search with filter or transaction
• Returning enough vectors is important
• Tired of tuning vectors.hnsw_ef_search in basic mode

You can enable vbase by a SQL statement SET vectors.search_mode = vbase;.

Prefilter

prefilter is an enhancement strategy that can be set in any search mode. It is enabled by default.

If enabled, an additional filter is applied to select vectors before they are collected. This reduces expensive distance calculations, and all collected vectors will match the filter.

The acceleration ratio is positively correlated to the filtering rate. A filtering rate of 10% in will result in a 10x acceleration of distance calculations.

Details

Example: 10% filtering rate

SELECT * FROM generate_series(1, 10) WHERE generate_series <= 1;

However, prefilter can have a negative impact on precision if:

• The filter is not relevant to the vector distance
• The filtering rate is too low, e.g. 1%.

Details

Example: 1% filtering rate

SELECT * FROM generate_series(1, 100) WHERE generate_series <= 1;

If you need a high level of precision, please test your scenarios and consider turning it off:

ALTER SYSTEM SET vectors.enable_prefilter=off;