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pgvector adds vector storage and similarity search to PostgreSQL. It lets you keep embeddings beside ordinary relational data and query the nearest records with SQL. PostgreSQL is enough when tests on your own data show acceptable relevance, latency, filtering behavior, throughput, and operating cost. There is no documented row-count cutoff that says when you must move to a separate vector database.

What pgvector adds to PostgreSQL

pgvector is a PostgreSQL extension, not a replacement database. It supplies vector data types, distance operators, and indexes for nearest-neighbor searches, while PostgreSQL continues to manage tables, SQL queries, and relational data. That can let an application retrieve similar records and apply ordinary database filters within the system it already operates.

A typical nearest-neighbor query orders rows by a distance operator and limits the results. The distance metric and vector representation should match the way the application creates and uses its embeddings; a mathematically close vector is not, by itself, proof that a result is useful to a person.

Choose between exact search and approximate indexes

Exactness and approximation are the first important decision. The pgvector project documentation says, “By default, pgvector performs exact nearest neighbor search, which provides perfect recall.” Exact search compares against eligible stored vectors using the selected distance calculation. It does not guarantee that the underlying embeddings capture the meaning or relevance your application needs.

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HNSW and IVFFlat indexes can make nearest-neighbor searches faster by considering a subset of candidates, but may omit true nearest neighbors. Whether that tradeoff is acceptable depends on measured query speed and recall for your workload.

HNSW

HNSW organizes vectors in a multilayer graph. The project describes it as offering a better speed–recall tradeoff than IVFFlat, at the cost of slower index builds and greater memory use. It can be built before loading data because it does not require a training step. Its documented default hnsw.ef_search is 40; increasing search effort can improve recall while taking more time.

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Understand filters, result counts, and tenants

With approximate indexes, filtering is applied after the vector index scan. As a result, a query asking for a fixed number of nearest records in a category or tenant can return fewer qualifying rows than requested. The pgvector documentation illustrates the effect: if a filter matches 10% of rows and HNSW uses its default search breadth of 40, about four matches would be expected on average before further scanning. This is an explanatory estimate, not a benchmark or guarantee for a particular dataset.

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For a selective filter, an ordinary PostgreSQL index on the filter column may make exact search over the eligible subset a practical choice. If an approximate index is needed, pgvector documents several options:

  • Iterative scans: Available from pgvector 0.8.0, these can continue scanning until enough qualifying results are found or a configured limit is reached. Strict ordering preserves exact distance order; relaxed ordering can improve recall while allowing slight reordering.
  • Partial indexes: Consider them when there are only a few distinct filter values and each has a useful subset of the data.
  • Partitioning: Consider it when there are many distinct values or when query patterns make separate subsets appropriate. The documentation warns that tenants sharing an approximate index can affect one another’s recall and speed; list partitioning or separate tables may provide isolation.

Keep conventional indexes on commonly used filter columns where appropriate, and test realistic filter selectivity. An index strategy that works for unfiltered nearest-neighbor queries may behave differently for category- or tenant-scoped retrieval.

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Combine vector retrieval with text search when it helps

PostgreSQL full-text search can be combined with pgvector for hybrid retrieval. The pgvector documentation points to Reciprocal Rank Fusion and cross-encoders as ways to combine or rerank results in application logic. These are techniques to evaluate, not automatic improvements: compare task-level relevance on representative queries before adopting one.

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Plan for data loading, storage, and index operations

For bulk ingestion, the project recommends using PostgreSQL COPY and adding indexes after the initial load. In production, concurrent index creation can avoid blocking writes. HNSW vacuum work may be lengthy; the documentation suggests reindexing concurrently before vacuuming.

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For smaller representations, pgvector supports halfvec and binary quantization with reranking. Reduced precision or quantization brings representation and recall tradeoffs, so evaluate the resulting quality and storage footprint against the application’s requirements. Type dimension limits are version-sensitive: the current project README lists limits of 2,000 dimensions for vector, 4,000 for halfvec, and 64,000 for bit. Check the documentation for the pgvector release you install before relying on those limits.

Measure whether PostgreSQL is enough

Do not decide based on a generic row-count slogan. pgvector’s documentation does not establish a universal row threshold, and no single database choice follows from the number of vectors alone. Benchmark representative data, queries, concurrency, filters, and update patterns on the hardware and PostgreSQL configuration you plan to run.

  1. Establish a quality baseline. Run exact search for representative queries and define what recall and task-level relevance your application requires.
  2. Measure approximate search against that baseline. Compare HNSW or IVFFlat results with exact results while varying the search settings and testing the real filters your application uses.
  3. Inspect query costs and latency. Use EXPLAIN (ANALYZE, BUFFERS) to examine query execution and buffer activity, and track latency and throughput at expected concurrency.
  4. Include operational work. Measure ingestion, index-build time, updates, backups, recovery, and memory and storage needs—not just query speed.
  5. Change the architecture only when evidence points to it. PostgreSQL can be scaled with more memory, CPU, and storage on one instance, replicas, or sharding approaches. Evaluate these options against the team’s operational constraints.

When to consider a separate retrieval system

Consider another retrieval system if a representative comparison shows that PostgreSQL cannot meet a material requirement—such as recall, task-level relevance, p50 or p95 latency, throughput, filtered search behavior, tenant isolation, or operational cost—and the alternative performs better for that requirement in your workload. Compare systems using the same query set and data, and include ingestion, updates, index footprint, backups, recovery, hybrid retrieval, and the team’s ability to operate each option. The pgvector documentation does not provide cross-vendor benchmarks, so it cannot establish a universal winner.

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