How to calculate how too much memory ram need a PostgreSQL Server?

Hi folks!!

I support a PostgreSQL database it grew from 100 to 300 GB in couple months,
I we need tuning all OS parameters (is a virtual enviroment). Currently the
configuration is the follow:

Virtual Machine
Ubuntu 18.04
12 CPUS
8 GB in RAM
400 in Disk
Clúster database size 320 GB

Some queries have times too differents, the same query sometimes it execute
in 8 10 seconds sometimes in 12 15 minutes, many querys make joins between
40, 50 millions rows. I know some factors like relation between entities
affect times, but in general I suspect the mainly problem si resources in
OS.

I suspect is a lack to memory ram but we need justify the change in OS RAM,
so, How to calculate how too much memory ram need a PostgreSQL Server? Too
need know how because the database size continue grows.

Best Regards
DrakoRod

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Dame un poco de fe, eso me bastará.
Wolfgres - Postgres Enterprise
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