Every team building a RAG system in 2026 faces the same decision tree. You need a vector store. You open the comparison guides. Pinecone says it is the fastest. Milvus says it scales to billions. pgvector says you already have PostgreSQL. Neo4j says relationships matter. FAISS says nothing because it is a library that does not make claims.
None of them are wrong. All of them are incomplete answers.
The right vector store is determined by one thing and one thing only: the shape of the queries your users actually ask. Not your current query distribution — your full query distribution, including the hard queries you have not written good answers for yet.
This is the complete guide to what each option actually does, where each one genuinely wins, and — critically — why Neo4j occupies a fundamentally different architectural position than every other option on this list.
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