Re: 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 <rakeshkumar464a3@gmail.com>
> Cc: PostgreSQL General <pgsql-general@postgresql.org>
> 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 <a.ignatov@postgrespro.ru> 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.
Forexample, if you recently upgraded from release 11.2.0.4 to Oracle Database 12c, and you did not change the
compatibleinitialization 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
newerfeatures. 
>
> 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