--- title: Efficient Similarity Search using Vector Indexes | daily.dev description: Covers approximate nearest neighbor (ANN) vector search as an alternative to exact vector similarity search once data volume grows, using vector indexes in SQL... canonical: https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Efficient Similarity Search using Vector Indexes | daily.dev og:description: Covers approximate nearest neighbor (ANN) vector search as an alternative to exact vector similarity search once data volume grows, using vector indexes in SQL... og:url: https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix og:image: https://api.daily.dev/og/posts/2rAZP6uIX.png og:image:alt: Efficient Similarity Search using Vector Indexes og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Efficient Similarity Search using Vector Indexes **[NikolaTech](https://daily.dev/sources/nikolatech)** · 5 min read · 0 upvotes · 0 comments ## Summary Covers approximate nearest neighbor (ANN) vector search as an alternative to exact vector similarity search once data volume grows, using vector indexes in SQL Server 2025 with EF Core. Walks through mapping a VECTOR(1024) column, creating a DiskANN-style cosine index via raw SQL (since HasVectorIndex() and VectorSearch() aren't yet exposed in the EF Core API), and querying with SQL Server's VECTOR_SEARCH table-valued function through SqlQueryRaw. Notes that tables become read-only while a vector index exists, and recommends exact search for small datasets or when optimal correctness is required, moving to approximate indexed search when latency and scale matter. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How do I create a vector index in SQL Server 2025 since EF Core doesn't support HasVectorIndex() yet? Create the index with raw SQL rather than EF Core's API, since HasVectorIndex() and VectorSearch() are still in development and not yet exposed. Use a statement like CREATE VECTOR INDEX vec_idx ON Documents(Embedding) WITH (METRIC = 'cosine', TYPE = 'diskann'). The table must have a clustered primary key on a single INT column, and it becomes read-only while the index exists. _daily.dev surfaces practical workarounds like this for developers tracking EF Core's evolving vector search support._ ### Why does my SQL Server table become read-only after creating a vector index? Current vector index behavior in SQL Server treats the table as read-only while the index exists, so writes aren't permitted alongside it. The recommended approach is to bulk-load data or finish writes first, then create the index; if data must change later, drop the index, update rows, and recreate it. _Developers planning data pipelines around vector indexes can keep up with these constraints via daily.dev._ ### When should I use exact vector search instead of approximate (ANN) search? Exact vector search, which computes distance between the query vector and every row, is best for small datasets or when correctness demands the single mathematically closest match. Approximate nearest neighbor search backed by vector indexes is better when latency and scale matter, trading a marginally lower chance of the optimal match for much faster, more scalable queries. _daily.dev helps developers weighing exact versus approximate search trade-offs stay current on the techniques._ ## Similar posts on daily.dev - [DiskANN Vector Index and Vector Search Are Now Generally Available in Azure SQL](https://daily.dev/posts/diskann-vector-index-and-vector-search-are-now-generally-available-in-azure-sql-osouki4it) · DevBlogs · 0 upvotes · 0 comments - [Vector Index DiskANN Embeddings for AI Solutions](https://daily.dev/posts/vector-index-diskann-embeddings-for-ai-solutions-ghudmc57k) · DevBlogs · 0 upvotes · 0 comments --- Tags: [#c#](https://daily.dev/tags/c#), [#microsoft-sql-server](https://daily.dev/tags/microsoft-sql-server), [#vector-search](https://daily.dev/tags/vector-search), [#embeddings](https://daily.dev/tags/embeddings) [View this post on daily.dev](https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Efficient Similarity Search using Vector Indexes","url":"https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix"},"datePublished":"2026-10-04T13:07:44.933Z","dateModified":"2026-10-04T13:19:23.087Z","description":"Covers approximate nearest neighbor (ANN) vector search as an alternative to exact vector similarity search once data volume grows, using vector indexes in SQL...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/0d5345c154f993cc302ebe81a15d2a50?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/0d5345c154f993cc302ebe81a15d2a50?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"NikolaTech","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"NikolaTech","logo":"https://media.daily.dev/image/upload/s--1h2fO6FL--/f_auto,q_auto/v1791119050/logos/nikolatech?_a=BAMAMicg0","url":"https://daily.dev/sources/nikolatech"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"c#,microsoft-sql-server,vector-search,embeddings","timeRequired":"PT5M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"NikolaTech","item":"https://daily.dev/sources/nikolatech"},{"@type":"ListItem","position":3,"name":"Efficient Similarity Search using Vector Indexes"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/efficient-similarity-search-using-vector-indexes-2razp6uix#faq","mainEntity":[{"@type":"Question","name":"How do I create a vector index in SQL Server 2025 since EF Core doesn't support HasVectorIndex() yet?","acceptedAnswer":{"@type":"Answer","text":"Create the index with raw SQL rather than EF Core's API, since HasVectorIndex() and VectorSearch() are still in development and not yet exposed. 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