| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Startup cost of sequential scan |
| Date: | 2018-08-30 21:38:36 |
| Message-ID: | 26910.1535665116@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Whose mental model? I guess the Tom Lane mind is the canonical one
> for this project, but I'm not sure that it entirely agrees with mine.
Since the fact that we have a notion of startup cost at all is entirely my
fault, I don't feel shy about claiming to have the authoritative view of
what it means.
(Whether that's adequately documented is another question :-()
> IIRC, it was previously proposed that we ought to charge
> random_page_cost for the first block of a sequential scan, because at
> present the cost of fetching 1 block differs depending on whether we
> are fetching it from a heap or an index, which seems unprincipled.
That might well be a sane thing to do ... but it'd still be part of run
cost not startup cost.
regards, tom lane
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