pg 中各类时间戳处理及显示

SELECT now() as "System Time",
now()::timestamp as "postgres Time",
now() AT TIME ZONE 'GMT' as "time without zone",
now() AT TIME ZONE 'CST' as "time without zone",
now()::timestamp at TIME ZONE 'GMT' as "Timestamp GMT",
now()::timestamp at TIME ZONE 'CST' as "Timestamp CST" ;