Milvus Vector - Unable to Set `nprobe` in `doSimilaritySearch` for Default `IVF_FLAT` When `initializeSchema = true`

[waileong]

Milvus Vector - Unable to Set nprobe in doSimilaritySearch for Default IVF_FLAT When initializeSchema = true

Description

We are encountering an issue in MilvusVectorStore where it is not possible to set nprobe when performing similarity searches (doSimilaritySearch) if the index type is IVF_FLAT.

By default, IVF_FLAT is selected when initializeSchema = true, but nprobe is not explicitly set, leading to poor recall or zero results in some cases.

Expected Behavior

• Users should be able to set nprobe when performing similarity searches with IVF_FLAT.
• A way to override nprobe in doSimilaritySearch() should be provided via either:
  • A configurable parameter in SearchRequest
  • A fallback to a reasonable default value (e.g., nprobe = 256)

Current Behavior

• IVF_FLAT is used as the default index, but nprobe is not explicitly set in doSimilaritySearch().
• This leads to low recall or 0 search results if nprobe defaults to a very small value (1).
• There is no exposed method to override nprobe in SearchRequest.

Steps to Reproduce

1. Initialize a MilvusVectorStore instance with initializeSchema = true.
2. Add some vector data to the store.
3. Perform a similarity search using doSimilaritySearch().
4. Observe that nprobe is not explicitly set, leading to poor recall or zero results.

Code Example:

MilvusVectorStore vectorStore = MilvusVectorStore.builder(milvusClient, embeddingModel)
    .initializeSchema(true)  // Uses default IVF_FLAT
    .build();

SearchRequest searchRequest = SearchRequest.query("example query")
    .withTopK(5)
    .withSimilarityThreshold(0.7);  // No way to set nprobe

List<Document> results = vectorStore.similaritySearch(searchRequest);

// Expecting search results, but sometimes returns 0 records due to missing nprobe

[dev-jonghoonpark]

related document : https://milvus.io/docs/ivf-flat.md

[dev-jonghoonpark]

To achieve this feature request,
we need to add a field for custom params to the SearchRequest class.
but this change would affect all vector stores.

[waileong]

This is not just a feature request; it's a necessary functionality because all index types in Milvus require custom parameters for optimal performance.

• IVF-based indexes (e.g., IVF_FLAT, IVF_PQ) require nprobe to control the number of clusters searched.

  • Without explicitly setting nprobe, the recall is significantly impacted.
  • The default value (often nprobe=1) leads to poor search results, sometimes returning zero results.

• HNSW indexes require ef for search expansion.

  • Just like nprobe, ef must be configurable for optimal performance.

• The lack of a way to configure nprobe means users are stuck with suboptimal defaults, which defeats the purpose of using an optimized vector database like Milvus.

• Since IVF_FLAT is the default index when initializeSchema = true, the inability to set nprobe directly in SearchRequest severely limits its usability.

• A proper solution should allow users to specify index-specific parameters within SearchRequest, ensuring flexibility and control over Milvus indexing.

For more details on why these parameters are essential, refer to the Milvus documentation:
https://milvus.io/docs/index.md?tab=floating.

[waileong]

@ilayaperumalg, I would be grateful if you could kindly look into this issue. Thank you very much.

[ilayaperumalg]

@waileong Thank you for your contribution! The changes are merged as e128a39