查看会话数

1. 使用 pg_stat_activity 系统视图（最常用）
   sql
   -- 查看所有活动会话
   SELECT COUNT(*) as total_connections FROM pg_stat_activity;

-- 查看详细信息
SELECT
datname as database,
usename as username,
client_addr as client_address,
state,
count(*) as connections
FROM pg_stat_activity
WHERE pid <> pg_backend_pid() -- 排除当前查询会话
GROUP BY datname, usename, client_addr, state
ORDER BY connections DESC;

-- 查看不同状态的会话数
SELECT
state,
count(*)
FROM pg_stat_activity
GROUP BY state;