Re: bgwriter, inherited temp tables TODO items?

Great! Is background writer clogging worthy? That's the one that put
postgres in a nearly unusable state after this bug was tripped.

Thanks!

--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC

Strategic Open Source: Open Your i™

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On Jul 29, 2005, at 10:49 PM, Bruce Momjian wrote:

> Added to TODO:
>
> * Prevent inherited tables from expanding temporary subtables
> of other
> sessions
>
>
> ----------------------------------------------------------------------
> -----
>
> Thomas F. O'Connell wrote:
>
>>
>> On Jul 21, 2005, at 1:22 PM, Bruce Momjian wrote:
>>
>>
>>> Thomas F. O'Connell wrote:
>>>
>>>
>>>> I'm switching the aftermath of this thread -- http://
>>>> archives.postgresql.org/pgsql-general/2005-07/msg00501.php -- to -
>>>> hackers since it raised issues of potential concern to developers.
>>>>
>>>> At various points in the thread, Tom Lane said the following:
>>>>
>>>> "I have an old note to myself that persistent write errors could
>>>> "clog"
>>>> the bgwriter, because I was worried that after an error it would
>>>> stupidly try to write the same buffer again instead of trying to
>>>> make
>>>> progress elsewhere. (CVS tip might be better about this, I'm not
>>>> sure.)
>>>> A dirty buffer for a file that doesn't exist anymore would
>>>> certainly
>>>> qualify as a persistent failure."
>>>>
>>>> and
>>>>
>>>> "Hmm ... a SELECT from one of the "actual tables" would then
>>>> scan the
>>>> temp tables too, no?
>>>>
>>>> Thinking about this, I seem to recall that we had agreed to make
>>>> the
>>>> planner ignore temp tables of other backends when expanding an
>>>> inheritance list --- but I don't see anything in the code
>>>> implementing
>>>> that, so it evidently didn't get done yet."
>>>>
>>>> I don't immediately see TODO items correpsonding to these. Should
>>>> there be some? Or do these qualify as bugs and should they be
>>>> submitted to that queue?
>>>>
>>>>
>>>
>>> Would you show a query that causes the problem so I can properly
>>> word
>>> the TODO item for inheritance and temp tables?
>>>
>>
>> It's really more of a timing issue than a specific query issue.
>> Here's a scenario:
>>
>> CREATE TABLE parent ( ... );
>>
>> begin thread1:
>> CREATE TEMP TABLE child ( ... ) INHERITS FROM ( parent );
>>
>> begin thread2:
>> while( 1 ) {
>> SELECT ... FROM parent WHERE ...;
>> }
>>
>> end thread1 (thereby dropping the temp table at the end of session)
>>
>> At this point, the file is gone, but, as I understand it, the planner
>> not ignoring temp tables of other backends means that thread2 is
>> inappropriately accessing the temp table "child" as it performs
>> SELECTS, thus causing potential dirty buffers in bgwriter, which at
>> some point during the heavy activity of the tight SELECT loop, will
>> have the file yanked out from under it and will throw a "No such
>> file" error.
>>
>> So I guess the core issue is the failure of the planner to limit
>> access to temp tables.
>>
>> Tom seems to come pretty close to a TODO item in his analysis in my
>> opinion. Something like:
>>
>> "Make the planner ignore temp tables of other backends when expanding
>> an inheritance list."
>>
>> --
>> Thomas F. O'Connell
>> Co-Founder, Information Architect
>> Sitening, LLC
>>
>> Strategic Open Source: Open Your i?
>>
>> http://www.sitening.com/
>> 110 30th Avenue North, Suite 6
>> Nashville, TN 37203-6320
>> 615-260-0005
>>
>>
>
> --
> Bruce Momjian | http://candle.pha.pa.us
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