Обсуждение: Definition of random_page_cost seems to invert correlation of that setting and seq_page_cost

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Definition of random_page_cost seems to invert correlation of that setting and seq_page_cost

От
PG Doc comments form
Дата:
The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/runtime-config-query.html
Description:

While reading the following section in the documentation
https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-RANDOM-PAGE-COST
I found something which looks incorrect to me.

If I understand this setting correctly, the following sentence:

"Reducing this value relative to seq_page_cost will cause the system to
prefer index scans; raising it will make index scans look relatively more
expensive."

should rather be:

"Raising this value relative to seq_page_cost will cause the system to
prefer index scans; reducing it will make index scans look relatively more
expensive."

Thanks,
Nacho

Re: Definition of random_page_cost seems to invert correlation of that setting and seq_page_cost

От
Laurenz Albe
Дата:
On Mon, 2024-03-11 at 12:30 +0000, PG Doc comments form wrote:
> While reading the following section in the documentation
> https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-RANDOM-PAGE-COST
> I found something which looks incorrect to me.
>
> If I understand this setting correctly, the following sentence:
>
> "Reducing this value relative to seq_page_cost will cause the system to
> prefer index scans; raising it will make index scans look relatively more
> expensive."
>
> should rather be:
>
> "Raising this value relative to seq_page_cost will cause the system to
> prefer index scans; reducing it will make index scans look relatively more
> expensive."

The documentation is correct.  If the cost of a random I/O operation is
higher, index scans look more expensive, because they perform random I/O.

Yours,
Laurenz Albe

Re: [EXTERNAL] Re: Definition of random_page_cost seems to invert correlation of that setting and seq_page_cost

От
Nacho Alonso Portillo
Дата:
My bad. Misunderstood this when I read it. It's completely correct the way it is.

Thanks,
Nacho
From: Laurenz Albe <laurenz.albe@cybertec.at>
Sent: Wednesday, March 13, 2024 2:06 PM
To: Nacho Alonso Portillo <Ignacio.Alonso@microsoft.com>; pgsql-docs@lists.postgresql.org <pgsql-docs@lists.postgresql.org>
Subject: [EXTERNAL] Re: Definition of random_page_cost seems to invert correlation of that setting and seq_page_cost
 
[You don't often get email from laurenz.albe@cybertec.at. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]

On Mon, 2024-03-11 at 12:30 +0000, PG Doc comments form wrote:
> While reading the following section in the documentation
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.postgresql.org%2Fdocs%2Fcurrent%2Fruntime-config-query.html%23GUC-RANDOM-PAGE-COST&data=05%7C02%7Cignacio.alonso%40microsoft.com%7Cd97db203bd0749e7fb6008dc435e6cfc%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C638459320060756532%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=oBLi%2FJJPxpSmzEYmy8adBFZjSHJWq%2F6OIP%2BEvR25Vsk%3D&reserved=0
> I found something which looks incorrect to me.
>
> If I understand this setting correctly, the following sentence:
>
> "Reducing this value relative to seq_page_cost will cause the system to
> prefer index scans; raising it will make index scans look relatively more
> expensive."
>
> should rather be:
>
> "Raising this value relative to seq_page_cost will cause the system to
> prefer index scans; reducing it will make index scans look relatively more
> expensive."

The documentation is correct.  If the cost of a random I/O operation is
higher, index scans look more expensive, because they perform random I/O.

Yours,
Laurenz Albe