FastAPI - Database Integration
Given this FastAPI code snippet using SQLAlchemy with connection pooling, what happens when multiple requests come in simultaneously?
```python
engine = create_engine("postgresql://user:pass@localhost/db", pool_size=3, max_overflow=2)
SessionLocal = sessionmaker(bind=engine)
@app.get("/items")
async def read_items():
db = SessionLocal()
items = db.query(Item).all()
db.close()
return items
```