In Postgres, how do I adjust the "pgsql_tmp" setting?
From the manual


  temp_tablespaces (string)
  
  This variable specifies tablespaces in which to create temporary objects (temp tables and indexes on temp tables) when a CREATE command does not explicitly specify a tablespace. Temporary files for purposes such as sorting large data sets are also created in these tablespaces.


In order to be able to use that, you need to create a tablespace on that volume, create a directory there (owned by the postgres user!), then run this: 

create tablespace temp_space LOCATION 'mnt/volume-nyc1-01/pg_temp';


You need to grant the necessary privileges on that tablespace:

grant create on tablespace temp_space to public;


After that you can e.g. use 

alter system set temp_tablespaces=temp_space;


(alternatively you could just change that in your session)

Note that for the alter system to have any effect, you need to reload the configuration:

select pg_reload_conf();




Alternatively just change the $PGDATA/base/pgsql_tmp to be a symbolic link to the directory on the other partition.