Hybrid Search

Why Hybrid Search?

Pure vector search excels at semantic understanding but can miss exact term matches. Pure keyword search (BM25) excels at exact matches but misses synonyms and semantic intent. Hybrid search covers both failure modes:

Reciprocal Rank Fusion (RRF)

RRF merges two ranked lists by assigning each document a score based on its rank position in each list. Documents ranked highly in both lists receive the highest combined score:

public class HybridSearchService
{
    private const int RRF_K = 60;  // Standard RRF constant

    public async Task<IReadOnlyList<MemoryRecord>> SearchAsync(
        string collection,
        string queryText,
        float[] queryEmbedding,
        int topK,
        float minScore,
        CancellationToken ct)
    {
        // Run both searches in parallel
        var vectorTask  = _vectorStore.SearchAsync(collection, queryEmbedding, topK * 2, 0f, ct);
        var keywordTask = _keywordStore.SearchAsync(collection, queryText, topK * 2, ct);

        await Task.WhenAll(vectorTask, keywordTask);

        var vectorResults  = vectorTask.Result;
        var keywordResults = keywordTask.Result;

        // Build RRF score maps
        var scores = new Dictionary<string, double>();

        for (int rank = 0; rank < vectorResults.Count; rank++)
        {
            string id = vectorResults[rank].Id;
            scores[id] = scores.GetValueOrDefault(id) + 1.0 / (RRF_K + rank + 1);
        }

        for (int rank = 0; rank < keywordResults.Count; rank++)
        {
            string id = keywordResults[rank].Id;
            scores[id] = scores.GetValueOrDefault(id) + 1.0 / (RRF_K + rank + 1);
        }

        // Merge all records and sort by RRF score
        var allRecords = vectorResults.Concat(keywordResults)
            .GroupBy(r => r.Id)
            .Select(g => g.First())  // Deduplicate
            .ToList();

        return allRecords
            .OrderByDescending(r => scores[r.Id])
            .Take(topK)
            .ToList();
    }
}

Keyword Search Backend Options

Enabling Hybrid Search

// appsettings.json
{
  "Octopus": {
    "HybridSearch": {
      "Enabled":         true,
      "KeywordBackend":  "SqlServerFTS",  // SqlServerFTS | QdrantSparse | Elasticsearch
      "RrfK":            60
    }
  }
}

// Agent memory config
{
  "hybridSearchEnabled": true
}
// When enabled, SemanticMemoryService automatically routes
// through HybridSearchService instead of direct vector search.