OS cache management

Hello All, I would like to know about how OS cache works for postgres table
and index file.

Let's say I have 10 year data, and commonly used data only the last 1 year.
This data is quite big, so each table and index file is divided into
several file in PGDATA/base

Let's say 1 index named order_by_date has relfilenode = 1870772348, and
it's file consist of 1870772348, 1870772348.1, and 1870772348.2

And for oftenly queried 1 year data, do ALL files for the order_by_date
pushed to OS cache ? or it's just 1 file that contains index to this 1 year
data.

How about index named order_by_customer, will ALL the index files pushed to
OS cache ?

Can someone please explain about how OS cache works for this condition.

Thanks very much for the explanation.

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Regards,

Soni Maula Harriz