Hi Mark & Atri, :) Thanks for reply. But, I think i confused you. I am talking about access using indexes. So, I assume that B+ tree store key-value pair where rohit is the key and all the versions are its value.
Another way to think is I have a secondary index on emp. name and there are 4 rohit exist in DB. So, now B+ tree gives me 4 different tuple pointer for each Rohit. I want to know the code portion for this where i can see all 4 tuple pointer before each one have I/O access to fetch its tuple. Are the suggestions still valid? On Sun, Sep 14, 2014 at 10:53 AM, Mark Kirkwood < mark.kirkw...@catalyst.net.nz> wrote: > On 14/09/14 20:11, Rohit Goyal wrote: > >> Hi Mark, >> >> On Sun, Sep 14, 2014 at 8:57 AM, Mark Kirkwood >> <mark.kirkw...@catalyst.net.nz <mailto:mark.kirkw...@catalyst.net.nz>> >> wrote: >> > > 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 >> > > Well that's going to be a bit of a learning curve for you :-), the concept > to get familiar with is snapshots (see src/backand/access/heap/heapam.c > to get started). Thanks I will read it. :) Can you please tel me some specifics from this c file, if you already know :) > > > >> >> 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? >> > > I'm not sure we have specif examples in the docs for what you want to do, > but generally see http://www.postgresql.org/docs/9.4/static/plpgsql- > trigger.html > > Cheers > > Mark > -- Regards, Rohit Goyal
