pipelinedb 滑动窗口

内置的函数，总是返回当前的时间戳

事件达到的时间

CREATE CONTINUOUS VIEW recent_users WITH (sw = ‘1 minute‘) AS
   SELECT user_id::integer FROM stream;

相应的pipelinedb 会转换为：
CREATE CONTINUOUS VIEW recent_users AS
   SELECT user_id::integer FROM stream
WHERE (arrival_timestamp > clock_timestamp() - interval ‘1 minute‘);

CREATE CONTINUOUS VIEW sw0 WITH (sw = ‘1 hour‘) AS SELECT COUNT(*) FROM event_stream;
CREATE VIEW sw1 WITH (sw = ‘5 minutes‘) AS SELECT * FROM sw0;
CREATE VIEW sw2 WITH (sw = ‘10 minutes‘) AS SELECT * FROM sw0;