pgvector Index Not Being Used

How to fix it

// try these in order, the most common fix is first

1. FIX 1Fix the query shape

   Copy steps
   1. 01Use ORDER BY embedding <=> $1 LIMIT k directly on the table
   2. 02Avoid ORDER BY 1 - (embedding <=> ...) DESC
   Did this fix it? Yes No

2. FIX 2Match operator class

   1. 01Cosine needs vector_cosine_ops, L2 vector_l2_ops, inner product vector_ip_ops
   Did this fix it? Yes No

3. FIX 3Diagnose

   Copy steps
   1. 01Run EXPLAIN (ANALYZE, BUFFERS)
   2. 02Run ANALYZE on the table
   3. 03In a transaction try SET LOCAL enable_seqscan = off to see if cost is the issue
   Did this fix it? Yes No

Symptoms

EXPLAIN shows Seq Scan instead of an index scan.

Why it happens

• C-01Wrapped or descending ORDER BY
• C-02Distance computed in a subquery and sorted on alias
• C-03Operator class mismatch
• C-04Small table or stale stats

// this guide also covers

• pgvector index not used
• pgvector sequential scan
• pgvector explain analyze seq scan

Other pgvector errors

• pgvector Error: extension "vector" is not available
• pgvector HNSW Index Build Slow
• pgvector "operator does not exist: vector <=>" Error
• pgvector Dimension Limit: Cannot Index More Than 2000 Dimensions
• pgvector "different vector dimensions" Error
• pgvector Query Returns Fewer Results Than LIMIT With Filter
• pgvector HNSW Build Out of Memory or Shared Memory Error
• How to Enable pgvector on Supabase