Re: Bugs in TOAST handling, OID assignment and redo recovery

On Tue, Apr 10, 2018 at 7:24 PM, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
wrote:

> Hi Heikki,
>
> On Tue, Apr 10, 2018 at 7:07 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
> wrote:
>
>>
>>>
>> It would seem more straightforward to add a snapshot parameter to
>> GetNewOidWithIndex(), so that the this one caller could pass SnapshotToast,
>> while others pass SnapshotDirty. With your patch, you check the index
>> twice: first with SnapshotDirty, in GetNewOidWithIndex(), and then with
>> SnapshotToast, in the caller.
>>
>
> Hmm. I actually wrote my first patch exactly like that. I am trying to
> remember why I discarded that approach. Was that to do with the fact that SnapshotToast
> can't see all toast tuples either? Yeah, I think so. For example, it won't
> see tuples with uncommitted-xmin, leading to different issues. Now it's
> unlikely that we will have a OID conflict where the old tuple has
> uncommitted-xmin, but not sure if we can completely rule that out.
>

Or may be we simply err on the side of caution and scan the toast table
with SnapshotAny while looking for a duplicate? That might prevent us from
reusing an OID for a known-dead tuple, but should save us a second index
scan and still work.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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