no zuo no die系列，來自於pg的wiki。
這一節的內容是：不要使用current_time。
理由是:
>1.It returns a value of type timetz, for which see the previous entry. 
2.Use whichever of these is appropriate.

原因是：
1.該函式會返回timetz型別，而該型別已廢棄（應使用timestamptz）。
2.有更合適的函式可以使用
>CURRENT_TIMESTAMP or now() if you want a timestamp with time zone,
LOCALTIMESTAMP if you want a timestamp without time zone,
CURRENT_DATE if you want a date,
LOCALTIME if you want a time

current_time的使用
```
[local]:5432 pg12@testdb=# select * from pg_proc where proname = 'current_time';
(0 rows)

Time: 4.634 ms
[local]:5432 pg12@testdb=# \df current_time
                       List of functions
 Schema | Name | Result data type | Argument data types | Type 
--------+------+------------------+---------------------+------
(0 rows)

Time: 1.083 ms
[local]:5432 pg12@testdb=# \d current_time
Did not find any relation named "current_time".
[local]:5432 pg12@testdb=#

[local]:5432 pg12@testdb=# select * from current_time;
    current_time    
--------------------
 11:46:54.895344+08
(1 row)
```
從pg_proc和psql的df命令找不到該current_time。

Time: 4.780 ms
[local]:5432 pg12@testdb=# select now();
              now              
-------------------------------
 2019-10-21 11:51:40.708556+08
(1 row)

Time: 1.860 ms
[local]:5432 pg12@testdb=#

[local]:5432 pg12@testdb=# \df current_timestamp
                       List of functions
 Schema | Name | Result data type | Argument data types | Type 
--------+------+------------------+---------------------+------
(0 rows)

[local]:5432 pg12@testdb=# select * from pg_proc where proname = 'current_timestamp';
(0 rows)

Time: 3.014 ms
[local]:5432 pg12@testdb=# select * from current_timestamp;
      current_timestamp       
------------------------------
 2019-10-21 11:52:34.31964+08
(1 row)

Time: 2.246 ms
[local]:5432 pg12@testdb=# select current_timestamp;
       current_timestamp       
-------------------------------
 2019-10-21 11:52:37.363806+08
(1 row)

Time: 1.827 ms
[local]:5432 pg12@testdb=# 
```

2.LOCALTIMESTAMP
這是不帶時區的timestamp
```
[local]:5432 pg12@testdb=# \df localtimestamp
                       List of functions
 Schema | Name | Result data type | Argument data types | Type 
--------+------+------------------+---------------------+------
(0 rows)

[local]:5432 pg12@testdb=# select localtimestamp;
       localtimestamp       
----------------------------
 2019-10-21 11:53:43.376968
(1 row)

Time: 1.821 ms
[local]:5432 pg12@testdb=# 
```

3.CURRENT_DATE
返回當前日期（年月日）
```
[local]:5432 pg12@testdb=# \df current_date
                       List of functions
 Schema | Name | Result data type | Argument data types | Type 
--------+------+------------------+---------------------+------
(0 rows)

[local]:5432 pg12@testdb=# select current_date;
 current_date 
--------------
 2019-10-21
(1 row)

Time: 1.871 ms
[local]:5432 pg12@testdb=# 
```

4.LOCALTIME
不帶時區的當前時間（時分秒微秒）
```
[local]:5432 pg12@testdb=# \df localtime
                       List of functions
 Schema | Name | Result data type | Argument data types | Type 
--------+------+------------------+---------------------+------
(0 rows)

[local]:5432 pg12@testdb=# select localtime;
   localtime    
----------------
 11:54:45.38508
(1 row)

Time: 2.081 ms
[local]:5432 pg12@testdb=# 
```

**參考資料**
[Don't Do This](https://wiki.postgresql.org/wiki/Don't_Do_This)

PostgreSQL DBA(111) - pgAdmin(Don't do this：Don't use CURRENT_TIME)

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