| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Cc: | Haisheng Yuan <hyuan(at)pivotal(dot)io>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Bitmap table scan cost per page formula |
| Date: | 2017-12-20 22:18:52 |
| Message-ID: | CA+TgmoYeHxjoZGdWHrjw7Ltkw5LzViSWowQeHYcxqfn+yPnoHQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Dec 20, 2017 at 4:20 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>
> It is not obvious to me that the parabola is wrong. I've certainly seen
> cases where reading every 2nd or 3rd block (either stochastically, or
> modulus) actually does take longer than reading every block, because it
> defeats read-ahead. But it depends on a lot on your kernel version and
> your kernel settings and your file system and probably other things as well.
>
Well, that's an interesting point, too. Maybe we need another graph that
also shows the actual runtime of a bitmap scan and a sequential scan.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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