Subject: Re: Declarative partitioning
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 <573BC4DB.2040507@lab.ntt.co.jp> <14105.1463593739@sss.pgh.pa.us>
From: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Message-ID: <573D958D.4040308@lab.ntt.co.jp>
Date: Thu, 19 May 2016 19:29:33 +0900
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Ildar Musin <i.musin@postgrespro.ru>, pgsql-hackers@postgresql.org
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On 2016/05/19 2:48, Tom Lane wrote:
> Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> writes:
>> On 2016/05/18 2:22, Tom Lane wrote:
>>> The two ways that we've dealt with this type of hazard are to copy data
>>> out of the relcache before using it; or to give the relcache the
>>> responsibility of not moving a particular portion of data if it did not
>>> change.  From memory, the latter applies to the tuple descriptor and
>>> trigger data, but we've done most other things the first way.
> 
> After actually looking at the code, we do things that way for the
> tupledesc, the relation's rules if any, and RLS policies --- see
> RelationClearRelation().

I think I confused refcounting method of keeping things around with the
RelationClearRelation()'s method.  I now understand that you meant the
latter in your original message.

>> It seems that tuple descriptor is reference-counted; however trigger data
>> is copied.  The former seems to have been done on performance grounds (I
>> found 06e10abc).
> 
> We do refcount tuple descriptors, but we've been afraid to try to rely
> completely on that; there are too many places that assume a relcache
> entry's tupdesc is safe to reference.  It's not that easy to go over to
> a fully refcounted approach, because that creates a new problem of being
> sure that refcounts are decremented when necessary --- that's a pain,
> particularly when a query is abandoned due to an error.

I see.

>> So for a performance-sensitive relcache data structure, refcounting is the
>> way to go (although done quite rarely)?
> 
> I'd be suspicious of this because of the cleanup problem.  The
> don't-replace-unless-changed approach is the one that's actually battle
> tested.

OK, I will try the RelationClearRelation()'s method of keeping partition
descriptor data around so that no repeated copying is necessary.

Thanks,
Amit


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