Hi Mark, On Sun, Sep 14, 2014 at 8:57 AM, Mark Kirkwood < mark.kirkw...@catalyst.net.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. > > Exactly, one visible version is there per session. But, I want to test my algorithm in which i myself want to work on all the intermediate version and find the correct one for the session. > 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). > yes and I also want to know the portion of code where i can find all the historical or intermediate versions > > It is certainly possible (or used to be via snapshot manipulation, I > haven't looked at that code in a while sorry) to enable a session to see > all the old versions, and is quite a cool idea (Postgres used to have this > ability in older versions - called Time Travel). > > For pure practicality, this can be achieved without any code modifications > using TRIGGERs and an extra table (as Gavin alludes to). > Can you explain me more about how to starting working using trigger? > > Do tell us a bit more about what you are wanting to do! > > Cheers > > Mark > > -- Regards, Rohit Goyal
