As you are working with database, you should try to work as much as possible remotely, doing calculation and further transformation in-database.
Do you know about dplyr feature to work with DB thanks to dbplyr ?
See website : http://db.rstudio.com/dplyr/
With this, you'll be able to reduce the size of data that you need to collect for a graph or other R only operation.
dplyr on databases is very powerful, you should give a try to not do directly a SELECT *. Have you tried that already ?