| From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ANALYZE sampling is too good |
| Date: | 2013-12-06 00:23:11 |
| Message-ID: | CAGTBQpbXp9wrPnNm7QAGm5BTeorxyF0ce7-6hrBv9cYDC4iakQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Dec 3, 2013 at 8:30 PM, Greg Stark <stark(at)mit(dot)edu> wrote:
> Worse, my experience with the posix_fadvise benchmarking is that on
> spinning media reading one out of every 16 blocks takes about the same
> time as reading them all. Presumably this is because the seek time
> between tracks dominates and reading one out of every 16 blocks is
> still reading every track. So in fact if your table is up to about
> 3-4G ANALYZE is still effectively going to do a full table scan, at
> least as far as I/O time goes.
Actually, it's rotational latency the dominant cost there.
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