Re: Bitmap table scan cost per page formula

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От Robert Haas
Тема Re: Bitmap table scan cost per page formula
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Msg-id CA+TgmoYeHxjoZGdWHrjw7Ltkw5LzViSWowQeHYcxqfn+yPnoHQ@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Bitmap table scan cost per page formula  (Jeff Janes <jeff.janes@gmail.com>)
Ответы Re: Bitmap table scan cost per page formula
Re: Bitmap table scan cost per page formula
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On Wed, Dec 20, 2017 at 4:20 PM, Jeff Janes <jeff.janes@gmail.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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