Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Mon, Jul 18, 2016 at 9:10 PM, Noah Misch <noah@leadboat.com> wrote:
> On Sat, Jul 16, 2016 at 06:48:08PM -0400, Noah Misch wrote:
>> On Wed, Jul 13, 2016 at 03:57:02PM -0500, Kevin Grittner wrote:
>>> On Wed, Jul 13, 2016 at 12:48 PM, Andres Freund <andres@anarazel.de> wrote:
>>>> On 2016-07-13 10:06:52 -0500, Kevin Grittner wrote:
>>>>> On Wed, Jul 13, 2016 at 7:57 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>>>>>> On Tue, Jul 12, 2016 at 8:34 PM, Kevin Grittner <kgrittn@gmail.com> wrote:
>>>>>>> On Fri, Jul 8, 2016 at 1:52 PM, Andres Freund <andres@anarazel.de> wrote:
>>>>>>>
>>>>>>>> I'm a bit confused, why aren't we simply adding LSN interlock
>>>>>>>> checks for toast? Doesn't look that hard? Seems like a much more
>>>>>>>> natural course of fixing this issue?
>>>>>>>
>>>>>>> I took some time trying to see what you have in mind, and I'm
>>>>>>> really not "getting it".
>>>>>>
>>>>>> Isn't it possible if we initialize lsn and whenTaken in SnapshotToast
>>>>>> when old_snapshot_threshold > 0 and add a check for
>>>>>> HeapTupleSatisfiesToast in TestForOldSnapshot()?
>>>>>
>>>>> With that approach, how will we know *not* to generate an error
>>>>> when reading the chain of tuples for a value we are deleting.  Or
>>>>> positioning to modify an index on toast data.  Etc., etc. etc.
>>>>
>>>> I'm not following. How is that different in the toast case than in the
>>>> heap case?
>>>
>>> A short answer is that a normal table's heap doesn't go through
>>> systable_getnext_ordered().  That function is used both for cases
>>> where the check should not be made, like toast_delete_datum(), and
>>> cases where it should, like toast_fetch_datum().
>>>
>>> Since this keeps coming up, I'll produce a patch this way.  I'm
>>> skeptical, but maybe it will look better than I think it will.  I
>>> should be able to post that by Friday.
>>
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>
> IMMEDIATE ATTENTION REQUIRED.  This PostgreSQL 9.6 open item is long past due
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> [1] http://www.postgresql.org/message-id/20160527025039.GA447393@tornado.leadboat.com

As far as I can see, to do this the way that Andres and Amit
suggest involves tying in to indexam.c and other code in incredibly
ugly ways.  I think it is entirely the wrong way to go, as I can't
find a way to make it look remotely sane.  The question is whether
I should do it the way that I think is sane, or whether someone
else wants to show me what I'm missing by producing at least a
rough patch along these lines.

--
Kevin Grittner
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