无法检查Postgres DB aiohttp中是否存在用户

import asyncpg
from aiohttp import web

async def check_user(request):
    # 连接到Postgres数据库
    connection = await asyncpg.connect(
        user='your_username',
        password='your_password',
        database='your_database',
        host='your_host',
        port='your_port'
    )

    # 执行SQL查询
    query = "SELECT * FROM users WHERE username = $1"
    username = request.query.get('username')  # 从请求中获取用户名
    result = await connection.fetch(query, username)

    # 判断查询结果
    if len(result) > 0:
        return web.Response(text="用户存在")
    else:
        return web.Response(text="用户不存在")

app = web.Application()
app.router.add_get('/check_user', check_user)
web.run_app(app)