Prefiltering And Postfiltering since v0.4.0

VectorChord provides flexible filtering mechanisms to improve vector search performance and accuracy through prefiltering and postfiltering strategies. These filtering approaches determine when query conditions (like WHERE clauses) are applied during the vector similarity search process.

Structure

Prefiltering: Apply an additional filter before the search (original) step.

Postfiltering: Use the PostgreSQL native filter after the search (original) step.

Configuration

You can control the filtering strategy using the vchordrq.prefilter setting:

-- Enable prefiltering (default: off)
SET vchordrq.prefilter = on;

-- Use postfiltering  
SET vchordrq.prefilter = off;

WARNING

For complex queries, such as joint queries. Whether the prefilter takes effect is greatly affected by the planner/optimizer. Please evaluate carefully based on experiments.

Performance Trade-offs

Use prefiltering when:

• Your filtering conditions are highly selective (eliminate many candidates)
• You are using Rerank In Table index, while prefiltering can significantly reduce latency
• The filter is simple

Use postfiltering when:

• Your filtering conditions are less selective
• The filtering logic is complex and might benefit from having more candidates available
• The filter is a costly operation

Example	All rows	Selected rows	Select rate
A low selective filter	1000	900	90%
A medium selective filter	1000	300	30%
A highly selective filter	1000	10	1%

WARNING

If your filter is not a pure function and have some side effects, vchordrq.prefilter could cause a change in behavior.

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Based on our experimental results, the QPS speedup at different select rate is as follows:

• 200% speedup at a select rate of 1%
• Not significant (5%) speedup at a select rate of 10%