Re: preserving forensic information when we freeze

On 2014-01-02 07:35:11 -0500, Robert Haas wrote:
> On Thu, Jan 2, 2014 at 12:26 AM, Greg Stark <stark(at)mit(dot)edu> wrote:
> >> I fail to see why. What is so ugly about this:
> >
> >> select x.* from pgbench_accounts a, pg_tuple_header(a.tableoid, a.ctid) x;
> >
> > Two points:
> >
> > 1) it's a bit weird to go to this effort to eliminate system columns by
> > using a scheme that depends on having a system column -- ctid
> >
> > 2) refetching a row could conceivably end up retrieving different data than
> > was present when the row was originally read. (In some cases that might
> > actually be the intended behaviour)
> >
> > If this came up earlier I'm sorry but I suppose it's too hard to have a
> > function like foo(tab.*) which magically can tell that the record is a heap
> > tuple and look in the header? And presumably throw an error if passed a non
> > heap tuple.
>
> Yeah, that was tried. Doesn't work:
>
> http://www.postgresql.org/message-id/CA+TgmoZWZZw_wr8aDHWc8iEjWEiHXLPXRaNqyrfb8mw0schEhg@mail.gmail.com
>
> At any rate, my goal isn't really to get rid of system columns, but to
> address Jim Nasby's gripe that the changes thus far committed make it
> difficult to use system columns to determine whether a given tuple has
> been frozen. It sounds like the consensus is that a system column is
> a better way of doing that than a function, so I suppose the next step
> is to try to hammer out the details.

So, I think something like my POC patch would need to get in before we
can go there, right? I won't be able to work on it in the next 10days or
so, there's a bunch of stuff that's more pressing unfortunately. There's
a fair bit of work left there...

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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