Psqlgresql Time function时间函数

select now() 获取的时间为什么一直不变？

pg中获取时间的方式有多种

如果放在事务中，now()获取的就是事务开始的时间，事务不结束，时间不变；而clock_timestamp()显示的时间会实时变化。获取时间行数参考如下：

now()
timestamp with time zone
Current date and time (start of current transaction); see Section 9.9.4

current_timestamp
timestamp with time zone
Current date and time (start of current transaction); see Section 9.9.4

current_time
time with time zone
Current time of day; see Section 9.9.4

clock_timestamp()
timestamp with time zone
Current date and time (changes during statement execution); see Section 9.9.4

eg：

postgres=# begin;
BEGIN
postgres=# select now();
              now              
-------------------------------
 2019-04-15 16:05:23.491101+08
(1 row)

postgres=# select now();
              now              
-------------------------------
 2019-04-15 16:05:23.491101+08
(1 row)

postgres=# end;
COMMIT
postgres=#


postgres=# begin;
BEGIN
postgres=# select clock_timestamp();
        clock_timestamp        
-------------------------------
 2019-04-15 16:04:20.491936+08
(1 row)
postgres=# select clock_timestamp();
        clock_timestamp        
-------------------------------
 2019-04-15 16:05:01.330757+08
(1 row)
postgres=# end;
COMMIT

reference

https://www.postgresql.org/docs/10/functions-datetime.html