# Creating an HNSW Index and Reading the Plan — Vector Databases

Source: https://www.geekswithgeeks.com/en/vector-databases/i-hnsw

> Add an index and confirm the planner uses it.

## One statement, new plan

`CREATE INDEX ... USING hnsw (embedding vector_l2_ops) WITH (m = 16, ef_construction = 64)` builds an **HNSW** graph index. The operator class must match your query operator: `vector_l2_ops` for `<->`, `vector_cosine_ops` for `<=>`, `vector_ip_ops` for `<#>`. Build parameters: **`m`** (links per node: more means better recall and a bigger index) and **`ef_construction`** (candidates examined while building: higher means a better graph and a slower build). After `ANALYZE`, `EXPLAIN` shows an `Index Scan using ...` with `Order By`, meaning the planner will use the ANN index. Build indexes **after** bulk loading (it is faster), allow time and memory for the build (`maintenance_work_mem` matters), and remember that an index returns **approximate** results.

## Build the index and check the plan, run

I ran this SQL on PostgreSQL 16 with the pgvector extension, version 0.8.6, in a Docker container. After `CREATE INDEX` and `ANALYZE`, the same query is planned as an `Index Scan using big_hnsw ... Order By (embedding <-> ...)` instead of a sequential scan and sort.

```sql
CREATE INDEX big_hnsw ON big USING hnsw (embedding vector_l2_ops) WITH (m = 16, ef_construction = 64);
ANALYZE big;
EXPLAIN (COSTS OFF)
SELECT id FROM big ORDER BY embedding <-> (SELECT embedding FROM big WHERE id = 1) LIMIT 10;
```

Output:

```
                   QUERY PLAN                   
------------------------------------------------
 Limit
   InitPlan 1 (returns $0)
     ->  Index Scan using big_pkey on big big_1
           Index Cond: (id = 1)
   ->  Index Scan using big_hnsw on big
         Order By: (embedding <-> $0)
(6 rows)
```

## Match the operator class

An index built with `vector_cosine_ops` is not used by an `<->` query. If `EXPLAIN` shows a sequential scan, check this first.

**Quiz:** How do you confirm that PostgreSQL uses the HNSW index?

- [x] EXPLAIN shows Index Scan using the index
- [ ] The table gets smaller
- [ ] The row count changes
- [ ] By restarting the server

*Answer:* EXPLAIN shows Index Scan using the index. The plan is the source of truth about what the planner does.