random_page_cost configuration parameter

Hello all,

the documentation of the random_page_cost configuration parameter says:

> Although the system will let you set random_page_cost to less than
> seq_page_cost, it is not physically sensible to do so.

Source: 
https://www.postgresql.org/docs/current/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS

However, I don't think this statement is true. Consider the situation 
where the randomly fetched pages are mostly (or even entirely) cached in 
RAM, but where the sequentially fetched pages must be mostly read from 
disk. An example for such a scenario is a database system which uses 
RAM-cached indices.

Such a database system would be best modeled with random_page_cost < 
seq_page_cost. Please correct me if I'm wrong.

Best regards
Mathias Kunter