Re: Postgres code for a query intermediate dataset

On Sun, Sep 14, 2014 at 1:30 PM, Mark Kirkwood <
mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> wrote:

> On 14/09/14 19:25, Atri Sharma wrote:
>
>>
>>
>> On Sunday, September 14, 2014, Mark Kirkwood
>> <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz <mailto:mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>>
>>
>> wrote:
>>
>> On 14/09/14 05:36, Rohit Goyal wrote:
>>
>> Hi All,
>>
>> I want to work on the code of intermediate dataset of select and
>> update
>> query.
>>
>> For example.
>>
>> Rohit's salary has been updated 4 times, so it has 4 different
>> version
>> of salary.
>>
>> I want to select salary of person named Rohit. Now suppose , in
>> intermediate result, I found 4 different versions of the data. I
>> want to
>> know the code portion which i need to look for working on all 4
>> versions
>> in dataset. :)
>>
>>
>>
>> Hi Rohit,
>>
>> Currently in Postgres, these intermediate versions all exist -
>> however a given session can only see one of them. Also VACUUM is
>> allowed to destroy versions that no other transactions can see.
>>
>> So if I'm understanding you correctly, you would like to have some
>> way for a session to see *all* these versions (and I guess
>> preventing VACUUM from destroying them).
>>
>>
>>
>> Any modifications of that sort are bound to introduce lots of pain, not
>> to mention performance degradation and the added responsibility of
>> ensuring that dead tuples don't bloat up the system (prevent vacuum from
>> running at regular intervals and you can have a xid wraparound).
>>
>> I just mentioned that in case you are planning to go in that direction.
>> If you only want the data, use the triggers as Gavin mentioned.
>>
>>
> Obviously in the general case sure - but (as yet) we don't have much idea
> about Rohit's use case and workload. If retrieving past versions is the
> *primary* workload bias and high update concurrency is not required then
> this could well work better than a trigger based solution.
>
> And it does not seem too onerous to have the ability to switch this on as
> required, viz:
>
> ALTER TABLE table1 VERSIONING;
>
> (or similar syntax) which makes VACUUM leave this table alone.
>

How do you plan to do all that VACUUM does for this table then?

It seems to me that you are saying to VACUUM that it need not be concerned
with table 'A' and you are assuming ownership of all the tasks performed by
VACUUM for this table. Seems pretty broken to me, not to mention the
performance degradations.

Regards,

Atri

Regards,

Atri