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This command will "su" the execution of the command to the postgres user(implies that you are already logger as root), and export the result of the query to a file on the csv format.
You'll need to adequate the fields and database information to one of your choice/need.
Get a listing of all of your databases in Postgres and their sizes, ordering by the largest size first.
Requires that you give the -d parameter a valid database name that you can connect to.