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Re: Update obsolete text in indexam.sgml

From: "Etsuro Fujita" <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "'PostgreSQL-development'" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Update obsolete text in indexam.sgml
Date: 2012-11-06 06:39:15
Message-ID: 003701cdbbe9$707a74e0$516f5ea0$@lab.ntt.co.jp (view raw or flat)
Thread:
Lists: pgsql-hackers
> From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]

> "Etsuro Fujita" <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> writes:
> > ISTM it would be better to update the text about index cost estimation in
> > indexam.sgml.  Please find attached a patch.
>
> I'm not too thrilled with the proposed patch.  In the first place, I
> don't think it's necessary to address costing of index order-by
> expressions in an introductory explanation.

Agreed.

> In the second, this change
> makes the code less clear, not more so, because it introduces a variable
> indexQuals without showing where you would get that value from.

Agreed.  However, I am concerned about the next comment in the current code:

/*
 * Our generic assumption is that the index pages will be read
 * sequentially, so they cost seq_page_cost each, not random_page_cost.
 * ...

I think this assumption is completely wrong, which has given me a motivation to
propose a patch, though I am missing something.

Thanks,

Best regards,
Etsuro Fujita




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