Re: Postgres code for a query intermediate dataset

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. It might
make more sense to make such a concept apply to a TABLESPACE instead
mind you (i.e things in here are for archive/versioning purposes)...

Clearly we'd need to see the code for any of this and evaluate if it is
good or terrible, but I'm not seeing the idea as bad as stated.

Cheers

Mark