| From: | Matthew Wakeling <matthew(at)flymine(dot)org> |
|---|---|
| To: | Kenneth Marshall <ktm(at)rice(dot)edu> |
| Cc: | Anj Adu <fotographs(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andy Colson <andy(at)squeakycode(dot)net>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: slow query performance |
| Date: | 2010-06-11 13:49:15 |
| Message-ID: | alpine.DEB.2.00.1006111434590.4083@aragorn.flymine.org |
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| Thread: | |
| Lists: | pgsql-performance |
On Fri, 11 Jun 2010, Kenneth Marshall wrote:
> If you check the archives, you will see that this is not easy
> to do because of the effects of caching.
Indeed. If you were to take the value at completely face value, a modern
hard drive is capable of transferring sequential pages somewhere between
40 and 100 times faster than random pages, depending on the drive.
However, caches tend to favour index scans much more than sequential
scans, so using a value between 40 and 100 would discourage Postgres from
using indexes when they are really the most appropriate option.
Matthew
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