To: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
From: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Subject: now() vs transaction_timestamp()
Message-ID: <6406dbd2-5d37-4cb6-6eb2-9c44172c7e7c@postgrespro.ru>
Date: Fri, 5 Oct 2018 16:29:28 +0300
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Postgres documentation says that |"now()| is a traditional PostgreSQL 
equivalent to |transaction_timestamp()|".
Also both use the same implementation.
But them have different parallel safety property:

postgres=# \df+ now
List of functions
    Schema   | Name |     Result data type     | Argument data types | 
Type | Volatility |  Parallel  |  Owner   | Security | Access privileges 
| Language |
  Source code |       Description
------------+------+--------------------------+---------------------+------+------------+------------+----------+----------+-------------------+----------+
-------------+--------------------------
  pg_catalog | now  | timestamp with time zone | | func | stable     | 
restricted | knizhnik | invoker |                   | internal |
  now         | current transaction time
(1 row)

postgres=# \df+ transaction_timestamp
List of functions
    Schema   |         Name          |     Result data type     | 
Argument data types | Type | Volatility | Parallel |  Owner   | Security 
| Access privileg
es | Language | Source code |       Description
------------+-----------------------+--------------------------+---------------------+------+------------+----------+----------+----------+----------------
---+----------+-------------+--------------------------
  pg_catalog | transaction_timestamp | timestamp with time zone 
|                     | func | stable     | safe     | knizhnik | 
invoker  |
    | internal | now         | current transaction time
(1 row)

As a result using now() in query disable parallel execution while 
transaction_timestamp allows it.
Was it done intentionally or it is just a bug?

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company


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    Postgres documentation says that <code class="FUNCTION">"now()</code>
    is a traditional <span class="PRODUCTNAME">PostgreSQL</span>
    equivalent to <code class="FUNCTION">transaction_timestamp()</code>".<br>
    Also both use the same implementation.<br>
    But them have different parallel safety property:<br>
    <br>
    postgres=# \df+ now  <br>
                                                                                        
    List of functions<br>
       Schema   | Name |     Result data type     | Argument data types
    | Type | Volatility |  Parallel  |  Owner   | Security | Access
    privileges | Language |<br>
     Source code |       Description        <br>
------------+------+--------------------------+---------------------+------+------------+------------+----------+----------+-------------------+----------+<br>
    -------------+--------------------------<br>
     pg_catalog | now  | timestamp with time zone |                    
    | func | stable     | restricted | knizhnik | invoker 
    |                   | internal |<br>
     now         | current transaction time<br>
    (1 row)<br>
    <br>
    postgres=# \df+ transaction_timestamp<br>
                                                                                               
    List of functions<br>
       Schema   |         Name          |     Result data type     |
    Argument data types | Type | Volatility | Parallel |  Owner   |
    Security | Access privileg<br>
    es | Language | Source code |       Description        <br>
------------+-----------------------+--------------------------+---------------------+------+------------+----------+----------+----------+----------------<br>
    ---+----------+-------------+--------------------------<br>
     pg_catalog | transaction_timestamp | timestamp with time zone
    |                     | func | stable     | safe     | knizhnik |
    invoker  |                <br>
       | internal | now         | current transaction time<br>
    (1 row)<br>
    <br>
    As a result using now() in query disable parallel execution while
    transaction_timestamp allows it.<br>
    Was it done intentionally or it is just a bug?<br>
    <br>
    <pre class="moz-signature" cols="72">-- 
Konstantin Knizhnik
Postgres Professional: <a class="moz-txt-link-freetext" href="http://www.postgrespro.com">http://www.postgrespro.com</a>
The Russian Postgres Company </pre>
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