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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Claudio Freire <klaussfreire@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>
Cc: Aleksander Alekseev <a.alekseev@postgrespro.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [Patch] Temporary tables that do not bloat pg_catalog (a.k.a fast temp tables)
Date: Wed, 24 Aug 2016 02:12:36 +0200
Message-ID: <58b5c129-5df4-0bf6-9468-ae119b6428d0@2ndquadrant.com> (raw)
In-Reply-To: <CAGTBQpZssJVSL897FUQmbzBJZSyzavbywQ2o_5BDES20Ei=ayA@mail.gmail.com>
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On 08/24/2016 12:38 AM, Claudio Freire wrote:
> On Tue, Aug 23, 2016 at 7:25 PM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
>>>> Could someone please explain how the unlogged tables are supposed to fix
>>>> the
>>>> catalog bloat problem, as stated in the initial patch submission? We'd
>>>> still
>>>> need to insert/delete the catalog rows when creating/dropping the
>>>> temporary
>>>> tables, causing the bloat. Or is there something I'm missing?
>>>
>>>
>>> Wouldn't more aggressive vacuuming of catalog tables fix the bloat?
>>>
>>> Perhaps reserving a worker or N to run only on catalog schemas?
>>>
>>> That'd be far simpler.
>>
>>
>> Maybe, although IIRC the issues with catalog bloat were due to a combination
>> of long queries and many temporary tables being created/dropped. In that
>> case simply ramping up autovacuum (or even having a dedicated workers for
>> catalogs) would not realy help due to the xmin horizon being blocked by the
>> long-running queries.
>>
>> Maybe it's entirely crazy idea due to the wine I drank at the dinner, but
>> couldn't we vacuum the temporary table records differently? For example,
>> couldn't we just consider them removable as soon as the backend that owns
>> them disappears?
>
> Or perhaps go all the way and generalize that to rows that never
> become visible outside their parent transaction.
>
> As in, delete of rows created by the deleting transaction could clean
> up, carefully to avoid voiding indexes and all that, but more
> aggressively than regular deletes.
>

Maybe, but I wouldn't be surprised if such generalization would be an 
order of magnitude more complicated - and even the vacuuming changes I 
mentioned are undoubtedly a fair amount of work.

Sadly, I don't see how this might fix the other issues mentioned in this 
thread (e.g. impossibility to create temp tables on standbys),


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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  Subject: Re: [Patch] Temporary tables that do not bloat pg_catalog (a.k.a fast temp tables)
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