PostgreSQL can do it on live data without importing anything. Set up the "file_fwd" extension:

  CREATE EXTENSION file_fdw;
  CREATE SERVER fdw FOREIGN DATA WRAPPER file_fdw;

Declare tables:

  CREATE FOREIGN TABLE person (id int, sex text)
    SERVER fdw
    OPTIONS (filename '/tmp/person.csv', format 'csv');
  CREATE FOREIGN TABLE weight (id int, weight int)
    SERVER fdw
    OPTIONS (filename '/tmp/weight.csv', format 'csv');

Now:

  SELECT * FROM person JOIN weight ON weight.id = person.id;