Re: Changing the default random_page_cost value

Bruce Momjian <bruce(at)momjian(dot)us> writes:
> On Thu, Oct 24, 2024 at 08:01:11PM -0400, Greg Sabino Mullane wrote:
>> Okay, but we have no documented logic on why 4.0 was chosen either. :)

> Uh, we do, and it is in the docs:

> Random access to mechanical disk storage is normally much more expensive
> than four times sequential access. However, a lower default is used
> (4.0) because the majority of random accesses to disk, such as indexed
> reads, are assumed to be in cache. The default value can be thought of
> as modeling random access as 40 times slower than sequential, while
> expecting 90% of random reads to be cached.

Meh. Reality is that that is somebody's long-after-the-fact apologia
for a number that was obtained by experimentation.

regards, tom lane