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Re: Largeobject Access Controls (r2460)

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>, Greg Smith <greg(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Largeobject Access Controls (r2460)
Date: 2009-12-19 01:58:32
Message-ID: 603c8f070912181758k70c78852gb32d9c41f162bb7d@mail.gmail.com (view raw or flat)
Thread:
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On Fri, Dec 18, 2009 at 1:48 AM, Takahiro Itagaki
<itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> wrote:
>> In both cases, I'm lost.  Help?
>
> They might be contrasted with the comments for myLargeObjectExists.
> Since we use MVCC visibility in loread(), metadata for large object
> also should be visible in MVCC rule.
>
> If I understand them, they say:
>  * pg_largeobject_aclmask_snapshot requires a snapshot which will be
>    used in loread().
>  * Don't use LargeObjectExists if you need MVCC visibility.

Part of what I'm confused about (and what I think should be documented
in a comment somewhere) is why we're using MVCC visibility in some
places but not others.  In particular, there seem to be some bits of
the comment that imply that we do this for read but not for write,
which seems really strange.  It may or may not actually be strange,
but I don't understand it.

...Robert

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