PL/R is a loadable procedural language that enables you to write PostgreSQL
functions and triggers in the R programming language. PL/R offers most (if
not all) of the capabilities a function writer has in the R language.

Commands are available to access the database via the PostgreSQL Server
Programming Interface (SPI) and to raise messages via elog() . There is no
way to access internals of the database backend. However the user is able
to gain OS-level access under the permissions of the PostgreSQL user ID,
as with a C function. Thus, any unprivileged database user should not be
permitted to use this language. It must be installed as an untrusted
procedural language so that only database superusers can create functions
in it. The writer of a PL/R function must take care that the function cannot
be used to do anything unwanted, since it will be able to do anything that
could be done by a user logged in as the database administrator.

An implementation restriction is that PL/R procedures cannot be used to
create input/output functions for new data types.