Search

Get the nearest 5 neighbors to a vector

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

Operators

These operators are used for distance metrics:

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

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

vchordrq.probes

• Description: This GUC parameter vchordrq.probes controls how the vector space assists in query pruning. The more probes, the more accurate the search, but also the slower it is.
• Type: list of integers
• Default:
• Example:
  • SET vchordrq.probes = 1 means that only one probe is used.
  • SET vchordrq.probes = 10 means that ten probes are used.
• Note: The default value is an empty list. The length of this option must match the length of lists.
  • If lists = [], then probes must be an empty list.
  • If lists = [11, 22], then probes can be 2,4 or 4,8, but it must not be an empty list, 3, 7,8,9, or 5,5,5,5.

vchordrq.epsilon

• Description: Even after pruning, the number of retrieved vectors remains substantial. The index employs the RaBitQ algorithm to quantize vectors into bit vectors, which require just the memory of single-precision floating-point vectors. With minimal floating-point operations, most computations are integer-based, leading to faster processing. Unlike typical quantization algorithms, RaBitQ not only estimates distances but also their lower bounds. The index computes the lower bound for each vector and dynamically adjusts the number of vectors needing recalculated distances, based on the query count, thus balancing performance and accuracy. The GUC parameter vchordrq.epsilon controls the conservativeness of the lower bounds of distances. The higher the value, the higher the accuracy, but the worse the performance. The default value provides unnecessarily high accuracy for most indexes, so you can try lowering this parameter to achieve better performance.
• Type: float
• Default: 1.9
• Domain: [0.0, 4.0]
• Example:
  • SET vchordrq.epsilon = 0.1 indicates you are using a very optimistic lower bound estimation. You set it this way because your dataset is not sensitive to the lower bound estimation, for the precision you need.
  • SET vchordrq.epsilon = 4.0 indicates you are using a very pessimistic lower bound estimation. You set it this way because your dataset is not very sensitive to the lower bound estimation, for the precision you need.
• Note: The default value is 1.9. The acceptable range is from 0.0 to 4.0.

You can refer to performance tuning for more information about tuning the query performance.

vchordrq.prefilter

• Description: The vchordrq.prefilter GUC parameter enables condition evaluation before distance computation. For example, in the query SELECT * FROM items WHERE id % 2 = 0 ORDER BY embedding <-> '[3,1,2]' LIMIT 5, the index normally computes all useful embedding <-> '[3,1,2]' distances first and then pass the rows to PostgreSQL, which filters out rows where id % 2 != 0. This parameter allows the index to pre-evaluate the condition and discard non-matching rows before computing their distances , improving query efficiency.
• Type: boolean
• Default: false

vchordrq.search_rerank

• Description: This GUC parameter controls the I/O prefetching strategy for reading bit vectors in vector search, which can significantly impact search performance on disk-based vectors.
• Type: string
• Domain: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: {"read_buffer", "prefetch_buffer"}
  • PostgreSQL 17: {"read_buffer", "prefetch_buffer", "read_stream"}
• Default: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: prefetch_buffer
  • PostgreSQL 17: read_stream
• Note:
  • read_buffer indicates a preference for ReadBuffer.
  • prefetch_buffer indicates a preference for both PrefetchBuffer and ReadBuffer.
  • read_stream indicates a preference for read_stream.

vchordrq.io_rerank

• Description: This GUC parameter controls the I/O prefetching strategy for reading full precision vectors in vector search, which can significantly impact search performance on disk-based vectors.
• Type: string
• Domain: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: {"read_buffer", "prefetch_buffer"}
  • PostgreSQL 17: {"read_buffer", "prefetch_buffer", "read_stream"}
• Default: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: prefetch_buffer
  • PostgreSQL 17: read_stream
• Note:
  • read_buffer indicates a preference for ReadBuffer.
  • prefetch_buffer indicates a preference for both PrefetchBuffer and ReadBuffer.
  • read_stream indicates a preference for read_stream.