postgresq與mysql中timestampdiff(SECOND,字段,NOW())相同求時間差值的秒值

postgresq與mysql中timestampdiff(SECOND,字段,NOW())相同求時間差值的秒值   mysql中寫法：  

select timestampdiff(SECOND,createtime,NOW()) from table1;

postgresql：

select extract(epoch FROM CURRENT_TIMESTAMP) - extract(epoch FROM createtime) from table1;