From: | Eric Ridge <eebbrr(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: How to get the 'ctid' from a record type? |
Date: | 2017-03-11 04:31:16 |
Message-ID: | CANcm6waPLx3KRR0rioBfMRn8kJwD_-+C4nZLDMueKr+u9dn-7Q@mail.gmail.com |
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Lists: | pgsql-hackers |
>
> I suspect the tuple at (0,1) has been the subject of a failed update.
>
Yep.
> Your problem here is that you're mistaking the t_ctid field of a tuple
> header for the tuple's address. It is not that; it's really just garbage
> normally, and is only useful to link forward to the next version of the
> row from an outdated tuple. I think we do initialize it to the tuple's
> own address during an INSERT, but either a completed or failed UPDATE
> would change it.
>
Thanks. That helps clarify the comments in htup_details.h, actually.
> I do not think there is any way to get the true address of a heap tuple
> out of a composite Datum manufactured from the tuple. Most of the other
> system columns can't be gotten from a composite Datum either, because of
> the field overlay in HeapTupleHeaderData's union t_choice.
Well shoot. That kinda spoils my plans.
What about this? Is the tuple currently being evaluated (I suppose in the
case of a sequential scan) available in the context of a function call?
Thanks again for your time! It's much appreciated.
eric
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