Re: Uber migrated from Postgres to MySQL

-----Original Message-----
From: Alex Ignatov [mailto:a.ignatov@postgrespro.ru] 
Sent: Thursday, July 28, 2016 11:26 AM
To: Igor Neyman ; Rakesh Kumar 
Cc: PostgreSQL General 
Subject: Re: [GENERAL] Uber migrated from Postgres to MySQL


On 28.07.2016 18:09, Igor Neyman wrote:
> -----Original Message-----
> From: pgsql-general-owner@postgresql.org 
> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Alex Ignatov
> Sent: Thursday, July 28, 2016 10:59 AM
> To: Rakesh Kumar 
> Cc: PostgreSQL General 
> Subject: Re: [GENERAL] Uber migrated from Postgres to MySQL
>
>
> On 28.07.2016 17:50, Rakesh Kumar wrote:
>> On Thu, Jul 28, 2016 at 10:38 AM, Alex Ignatov  wrote:
>>
>>> Sorry, what? You can rollback to previous version of software what you had.
>>>
>>> https://docs.oracle.com/cd/E11882_01/server.112/e23633/downgrade.htm
>>> #
>>> UPGRD007
>> Not so fast. This requires the db to be set in compatibility mode of 
>> the lower version. Once that is removed, no downgrade is possible.
>>
>> Also starting from Oracle 12 this downgrade option has been removed.
>> That is, you can't upgrade in-place from Or 11 to 12 and then 
>> downgrade back. The only supported way is the restore from an old 
>> backup (which is true  for all products). I don't work in Oracle, but 
>> got this clarified by someone who is an Oracle dba.
>>
>>
> Not true -
>
> https://docs.oracle.com/database/121/UPGRD/downgrade.htm#UPGRD007
>
> About compatible params you are right but downgrade is possible from 12c.
>
> Alex Ignatov
> Postgres Professional: http://www.postgrespro.com The Russian Postgres 
> Company
>
> ______________________________________________________________________
> _______
>
> Alex, documentation you point to just proves what Rakesh said:
>
> "For supported releases of Oracle Database, you can downgrade a database to the release from which you last upgraded. For example, if you recently upgraded from release 11.2.0.4 to Oracle Database 12c, and you did not change the compatible initialization parameter to 12.1 or higher, then you can downgrade to release 11.2.0.4."
>
> So, you can downgrade only if you didn't "activate" newer feature (didn't compatibility to higher version).
> But then, what's the point in upgrading (I know, there are some like bug fixes, etc...), if you aren't going to use newer features.
>
> Regards,
> Igor
Not true again.

 From Oracle docs
https://docs.oracle.com/database/121/UPGRD/intro.htm#UPGRD001 Section "How the COMPATIBLE Initialization Parameter Operates in Oracle Database"
-
If you run an Oracle Database 12c database with the COMPATIBLE initialization parameter set to 11.0.0, then it generates database structures on disk that are compatible with Oracle Database 11g. 
Therefore, the COMPATIBLE initialization parameter enables or disables the use of features. If you try to use any new features that make the database incompatible with the COMPATIBLE initialization parameter, then an error occurs. However, any new features that do not make incompatible changes on disk are enabled.

So if feature do not change disk in incompatible way this features is enable.

Also from this
https://docs.oracle.com/database/121/REFRN/GUID-6C57EE11-BD06-4BB8-A0F7-D6CDDD086FA9.htm#REFRN10019
:

"This parameter specifies the release with which Oracle must maintain compatibility. It enables you to take advantage of the maintenance improvements of a new release immediately in your production systems without testing the new functionality in your environment. Some features of the release may be restricted."

So this parameter is made with rollback in mind and you have some new feature from newer software release.

Alex Ignatov
Postgres Professional:http://www.postgrespro.com
The Russian Postgres Company

___________________________________________________________________________________________________________________

^^^
Which means that you can make use of some new feature, but definitely not all.
That makes "downgrade" feature very, very limited, if useful at all.

Regards,
Igor