| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> |
| Cc: | Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, pgsql-hackers(at)lists(dot)postgresql(dot)org |
| Subject: | Re: Startup cost of sequential scan |
| Date: | 2018-08-30 15:33:19 |
| Message-ID: | 9842.1535643199@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> The model (assuming I understand it rightly) is that what we're actually
> tracking is a startup cost and a per-output-row cost, but for comparison
> purposes we actually store the rows and the computed total, rather than
> just the per-row cost:
> rows
> startup_cost
> total_cost = startup_cost + (rows * per_row_cost)
Right. I tend to think of it differently: the cost to retrieve the
first K rows out of a total of N is
startup_cost + (total_cost - startup_cost) * K/N
but that comes out to the same thing as what Andrew said.
regards, tom lane
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