How do I extend pgvector with a custom distance metric?
Problem
I am interested in experimenting with a metric that is not included in pgvector on HNSW indices, can you provide a high level overview of what has to be done to support a new distance metric? I may be missing it, but a developer FAQ would be very helpful!
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1 Fix
Extend pgvector with Custom Distance Metric for HNSW
pgvector currently supports a limited set of distance metrics for HNSW indices. To implement a new metric, you need to modify the underlying C++ code of pgvector to define the new distance function and integrate it with the existing HNSW structure.
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- 1
Clone pgvector Repository
Start by cloning the pgvector GitHub repository to your local machine to access the source code.
bashgit clone https://github.com/yourusername/pgvector.git - 2
Define Custom Distance Metric
In the cloned repository, navigate to the file where distance metrics are defined (e.g., `distance.h`). Implement your custom distance function following the existing structure.
cpp// Example of a custom distance function float custom_distance(const float* a, const float* b, int dim) { // Implement your distance logic here } - 3
Integrate Metric into HNSW
Modify the HNSW index implementation to include your new distance metric. This may involve updating function pointers or conditionals that select the distance metric based on user input.
cpp// Example of integrating the custom distance if (metric == CUSTOM) { return custom_distance(a, b, dim); } - 4
Compile and Test Changes
Compile the modified pgvector code to ensure that your changes are correctly integrated. Run existing unit tests and add new tests for your custom metric to validate functionality.
bashmake && make test - 5
Document the Custom Metric
Update the developer documentation to include information about your new distance metric. This should cover how to use it, any parameters it requires, and examples.
markdown// Example documentation entry ## Custom Distance Metric Use the custom distance metric by specifying 'CUSTOM' in your query parameters.
Validation
To confirm the fix worked, run a series of queries using the new distance metric and compare the results against expected outcomes. Ensure that performance metrics are within acceptable ranges.
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Environment
Submitted by
Alex Chen
2450 rep