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Re: Dead Space Map version 2

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Jim C(dot) Nasby" <jim(at)nasby(dot)net>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Dead Space Map version 2
Date: 2007-02-28 09:51:46
Message-ID: 45E550B2.8030104@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patchespgsql-performance
Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
>> Tom Lane wrote:
>>> I'd be happier if the DSM content could be
>>> treated as just a hint.
> 
>> If we don't have a frozen state, we can't use the DSM to implement 
>> index-only scans.
> 
> To implement index-only scans, the DSM would have to be expected to
> provide 100% reliable coverage, which will increase its cost and
> complexity by orders of magnitude.  If you insist on that, I will bet
> you lunch at a fine restaurant that it doesn't make it into 8.3.

:)

While I understand that 100% reliable coverage is a significantly 
stronger guarantee, I don't see any particular problems in implementing 
that. WAL logging isn't that hard.

I won't insist, I'm not the one doing the programming after all. 
Anything is better than what we have now. However, I do hope that 
whatever is implemented doesn't need a complete rewrite to make it 100% 
reliable in the future.

The basic wish I have is to not use a fixed size shared memory area like 
FSM for the DSM. I'd like it to use the shared buffers instead, which 
makes the memory management and tuning easier. And it also makes it 
easier to get the WAL logging right, even if it's not done for 8.3 but 
added later.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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