pg vector example code

rds/pgvector/Readme.md

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+```sql
+SELECT * FROM pg_available_extensions WHERE name = 'vector';
+```
+
+```sql
+CREATE EXTENSION IF NOT EXISTS vector;
+```
+
+```sql
+CREATE TABLE items (
+    id SERIAL PRIMARY KEY,
+    embedding vector(3)  -- a vector of dimension 3
+);
+```
+
+```sql
+INSERT INTO items (embedding) VALUES 
+    ('[0.1, 0.2, 0.3]'),
+    ('[0.4, 0.5, 0.6]'),
+    ('[0.7, 0.8, 0.9]');
+```
+
+```sql
+SELECT * FROM items;
+```
+
+Nearest Neighbor Search
+
+```sql
+SELECT id, embedding, embedding <-> '[0.2, 0.1, 0.4]' AS distance
+FROM items
+ORDER BY embedding <-> '[0.2, 0.1, 0.4]' LIMIT 5;
+```