From: Tom Lane <tgl@sss.pgh.pa.us>
To: Manfred Koizar <mkoi-pg@aon.at>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Correlation in cost_index()
Date: Wed, 02 Oct 2002 18:48:49 -0400
Message-ID: <29475.1033598929@sss.pgh.pa.us> (raw)
In-Reply-To: <n6cmpug13b9rk1srebjvhphg0lm8dou1kn@4ax.com>
References: <n6cmpug13b9rk1srebjvhphg0lm8dou1kn@4ax.com>
Manfred Koizar <mkoi-pg@aon.at> writes:
> AFAICS (part of) the real problem is in costsize.c:cost_index() where
> IO_cost is calculated from min_IO_cost, pages_fetched,
> random_page_cost, and indexCorrelation. The current implementation
> uses indexCorrelation^2 to interpolate between min_IO_cost and
> max_IO_cost, which IMHO gives results that are too close to
> max_IO_cost.
The indexCorrelation^2 algorithm was only a quick hack with no theory
behind it :-(. I've wanted to find some better method to put in there,
but have not had any time to research the problem.
> As nobody knows how each of these proposals performs in real life
> under different conditions, I suggest to leave the current
> implementation in, add all three algorithms, and supply a GUC variable
> to select a cost function.
I don't think it's really a good idea to expect users to pick among
multiple cost functions that *all* have no guiding theory behind them.
I'd prefer to see us find a better cost function and use it. Has anyone
trawled the database literature on the subject?
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, mkoi-pg@aon.at
Subject: Re: Correlation in cost_index()
In-Reply-To: <29475.1033598929@sss.pgh.pa.us>
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