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Database / Qdrant Vector DB Interview questions

What is a collection in Qdrant?

A collection is the top-level container for a set of points (vectors plus their payloads) in Qdrant, roughly analogous to a table in a relational database, and it defines shared configuration like vector dimensionality, distance metric, and indexing/quantization settings that apply to every point stored in it.

from qdrant_client import QdrantClient, models

client = QdrantClient(url="http://localhost:6333")
client.create_collection(
    collection_name="documents",
    vectors_config=models.VectorParams(size=384, distance=models.Distance.COSINE),
)

Every point added to a collection must have a vector matching that collection's configured size and be compared using its configured distance metric, which is why the vector dimensionality and distance metric are fixed at collection-creation time rather than being flexible per point.

A collection can hold one or more named vectors per point (useful for storing multiple embeddings per item, like a text embedding and an image embedding side by side), and it's also the unit at which sharding, replication, and quantization settings are configured for a distributed deployment.

What is a collection roughly analogous to in a relational database?
What must every point added to a collection match?

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What is Qdrant? What is the purpose of Qdrant? What are the key features of Qdrant? What is a collection in Qdrant? What is a point in Qdrant? What is a payload in Qdrant? What is the HNSW algorithm? What distance metrics does Qdrant support? How do you create a collection in Qdrant? How do you insert/upsert points into a collection? What is the difference between REST and gRPC APIs in Qdrant? What client libraries are available for Qdrant? What is payload filtering in Qdrant? Define scalar quantization in Qdrant? What is a segment in Qdrant? How do you perform a similarity search in Qdrant? What is the purpose of a payload index? List the supported field types for payload indexing? What is Qdrant Cloud? What is memmap storage in Qdrant? What is the difference between Qdrant and Pinecone? What is the difference between Qdrant and Weaviate? Why is Qdrant implemented in Rust? How does the HNSW graph work internally in Qdrant? What is the difference between scalar, binary, and product quantization? Explain the internal working of binary quantization and why it's fast? What is oversampling and rescoring in quantized search? How does Qdrant implement filtering during HNSW traversal? Explain the internal working of Qdrant's sharding and replication? What consensus protocol does Qdrant use for distributed clusters, and how does it work? What are named vectors, and when should you use them? What are sparse vectors in Qdrant? Explain hybrid search using the Query API and Prefetch? What is Reciprocal Rank Fusion (RRF) versus Distribution-Based Score Fusion (DBSF)? How do you implement multitenancy in Qdrant? Explain the lifecycle of a write operation in Qdrant (WAL, segments, optimizers)? What is the role of the Write-Ahead Log (WAL) in Qdrant? How do you take and restore snapshots in Qdrant? What is the difference between keeping vectors on-disk versus the HNSW index in RAM? Explain the execution flow of a filtered vector search query in Qdrant? When should you choose binary quantization versus scalar quantization? How do you optimize Qdrant for high-throughput production workloads? What is the ACORN-1 method, and why does it matter for filtered search? Explain the internal working of Qdrant's segment optimizer/merging? How do you implement multi-vector (late interaction / ColBERT-style) search in Qdrant? What is the role of the payload index in query planning? How does Qdrant handle consistency during a node failure? Explain the execution flow of a RAG pipeline built with Qdrant as the retrieval layer? What are the trade-offs of self-hosting Qdrant versus using Qdrant Cloud? What is Qdrant's Discovery/Recommendation API used for?
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