What is a vector database?

A vector database stores embeddings, lists of numbers that capture what content means, and quickly finds the ones most similar to a query.

Concept

A vector database stores embeddings, lists of numbers that capture what content means, and quickly finds the ones most similar to a query. Think of it as a library catalog sorted by subject, not by title: ask for “more like this one” and it takes you straight to the right shelf. It keeps those vectors searchable as data changes, with the fast index, durable storage, updates, deletes, and filters a real application needs. That makes it a common retrieval layer behind search by meaning and chatbots that answer from your own documents.

Learning objectives

After reading this article you will be able to:

• Describe what a vector database stores and what it adds beyond a vector index
• Decide when an app needs a vector database, and where the vectors should live
• Explain where storing only vectors falls short for relationships and permissions
• Compare vector databases with graph databases and full-text search engines

What does a vector database store?

Put simply, each record typically holds a vector plus enough context to use it. For example, one chunk of a company wiki page might be stored as:

• An ID, such as doc-4821#chunk-3, which links the vector back to its source.
• A vector, an array of hundreds to thousands of numbers, used for similarity search.
• Metadata, such as the tenant (the customer or workspace that owns the data), the document type, and the created date, used for filtering and display.
• Content (optional), such as the chunk’s text, returned to the application or a language model.

The vector comes from an embedding model, typically run by the application before writing the record; some systems can call the model themselves. The embeddings explainer covers how those vectors are produced.

What does a vector database do?

It keeps vectors searchable while the data underneath keeps changing. Picture an online store adding products, editing descriptions, and retiring old items all day: each change has to show up in search.

• Writes. Inserts, updates, and deletes vectors while keeping the index current.
• Indexing. Builds an approximate nearest neighbor (ANN) index, such as HNSW or IVF, so a query does not compare against every vector. See What is vector search?
• Similarity queries. Takes a query vector and a number k and returns the k closest records it finds (approximately, when an ANN index is used), with their distances or scores.
• Filtering. Restricts results by metadata, such as a tenant or a date range, as covered in filtered vector search.
• Durability and scale. Persists data, recovers from failures, and serves many queries at once.

The term covers both dedicated systems built only for vectors and general-purpose databases that support vector indexes.

Do I need a vector database?

Not always. The answer depends on collection size, how often data changes, and where the source data already lives.

• Probably not for a small, static collection, such as a chatbot over one product manual. A few thousand vectors held in memory can be searched exhaustively, and a prototype rarely needs more.
• Probably yes when exhaustive search no longer meets your latency budget, when vectors change often, or when you need filters, durability, and concurrent access.
• Maybe not a separate one if your existing database supports vector indexes. The next section compares the two options.

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Should vectors live in a dedicated store or inside a general database?

It depends on how much retrieval relies on data that lives elsewhere. In other words, if search has to respect permissions, relationships, or live records, keeping vectors next to that data saves you from copying it around.

	Dedicated vector store	Vectors inside a general database
Where vectors live	A separate system	Next to the source records
Keeping in sync	A pipeline copies changes	Written with the record, in one transaction where supported
Filters	Metadata copied onto vectors	Conditions over the live data
Operations	One more system to run and secure	One system
Typical fit	Search isolated from other application data	Retrieval that depends on relationships, permissions, or transactional data

A separate store needs a pipeline that detects each change in the source data and copies the new vector or updated metadata into the store. Until the pipeline catches up, search can match stale content, return chunks of deleted documents, or honor revoked permissions. For example, someone removed from a project could keep seeing its documents in search results until the next sync. See Do you need separate graph, vector, and text databases? for these costs in more detail.

Where does vector-only storage fall short?

A vector captures what a piece of content is about, not how it connects to everything else. It does not capture:

• Relationships. Which chunk came from which document, which ticket belongs to which customer, which document cites which.
• Ownership and permissions. Who may see a record, often derived from team or group membership.
• Recency and lifecycle. Which version is current, and what was superseded or deleted.
• Provenance. Where a fact came from and which source to cite.

Vector stores usually approximate these by copying fields into each vector’s metadata. That works for flat, stable attributes such as a tenant ID or a document type. It works poorly for facts derived from relationships that change, such as group membership, sharing, or version history: one change can require rewriting metadata on many vectors. For example, when someone joins a team, the vector for every document that team can see may need its access list updated. A property graph models connections like these directly, as nodes and edges.

How does a vector database differ from a graph database or a search engine?

They answer different questions. A vector database finds what is similar, a graph database finds what is connected, and a full-text search engine finds what contains your words. Many systems now combine two or more of them.

	Vector database	Graph database	Full-text search engine
Question it answers	What is similar in meaning?	How are things connected?	Which documents contain these terms?
Core structure	Vectors and an ANN index	Nodes, edges, and properties	Inverted index over tokens
How results are selected	Ranked by distance between vectors	Matched by traversal or pattern; typically unranked or ordered by properties	Ranked by term statistics such as BM25
Weak at	Exact terms, relationships	Fuzzy meaning, unless it supports vectors	Paraphrase and synonyms

How are vector databases used in RAG?

In retrieval-augmented generation (RAG), a retriever finds passages to include in a language model’s prompt, and vector search over a vector database is the usual retriever. For example, a support chatbot embeds a customer’s question, retrieves the closest help-center passages, and hands them to the model to write an answer.

Anything retrieved can appear in the answer, so the gaps described above, especially permissions, freshness, and provenance, matter more in RAG than in a search box. The same goes for AI agent memory, where an agent retrieves its own past notes. A production retriever also typically needs:

• Exact term matching for names and IDs, often through BM25 keyword ranking combined with vectors in hybrid search.
• Related context, such as expanding a chunk to its document, author, or linked entities, as in GraphRAG.

How does HelixDB store vectors?

HelixDB is an open-source graph database with native vector search and BM25 full-text search. Vectors are stored as properties in the graph rather than in a separate store.

• An embedding is a property on a node or an edge, next to that entity’s other properties and relationships. The application computes embeddings; HelixDB stores and indexes them.
• A vector index covers one label and one top-level property, with a fixed dimension and a cosine, Euclidean, or Manhattan distance metric. Vector indexes can be partitioned by tenant.
• Each request is one ACID transaction, an all-or-nothing unit of work. A write batch that creates a document, its embedding, and its edges commits or rolls back as a unit, and vector search runs in the same transaction as graph traversals.
• Relationships such as ownership and provenance can be modeled as edges, so a vector search can be restricted to an exact candidate set defined by a traversal.

See the data model, vector indexes, and prefiltered search.

Frequently asked questions

Is a vector database the same as a vector index?

No. A vector index is a data structure, such as HNSW or IVF, that speeds up nearest neighbor search. A vector database wraps one or more indexes with storage, updates, filtering, durability, and a query interface.

Can a vector database enforce permissions?

Per-record permissions usually come from filters, though many systems can also isolate data in separate collections or tenant partitions. Filters typically check metadata copied onto each vector, such as a tenant or group ID, so that copy has to change whenever access changes. The filter must also apply before or during ranking; filtering after ranking can return fewer than k results. See filtered vector search.

Does a vector database store the original text?

It can, but it does not have to. Storing the chunk text with the vector lets results go straight to the application or a language model. Storing only an ID keeps a single copy of the text in the source system, at the cost of an extra lookup per result.

What happens when the embedding model changes?

Vectors from different models, or different versions of one model, are not comparable. Every stored vector has to be recomputed with the new model, the index rebuilt, and queries embedded with the same model. See What are vector embeddings?

Related topics

What is vector search?

Nearest neighbor search, recall, and exact vs approximate results.

What are vector embeddings?

How models turn text and images into vectors.

What is HNSW?

A widely used layered graph index for approximate search.

What is RAG?

Grounding a language model’s answer in retrieved data.

One database for graph, vector, and text

The case for keeping retrieval in one transactional system.

Vector indexes

Create a vector index in HelixDB.