Using untrusted Tcl (pltclu) is one of the oldest ways to do things outside the database. pltclu is executed using a normal Tcl interpreter and is pretty much free to do anything you'd like. Tcl has a lot of commands available to interact with the operating system and the environment. We will now take a look at a few simple examples.

The first example, reads the contents of the file and returns them as text, as shown in the following code:

CREATE OR REPLACE FUNCTION read_file(text) RETURNS text
AS $$
  set fptr [open $1]
  set file_data [read $fptr]
  close $fptr #close the file as it is already read
  return $file_data
$$ LANGUAGE pltclu;

The function is quite simple; it opens a file provided as a parameter, reads all its contents at once, and returns the text.

Let's run the preceding function:

postgres=# select read_file('/usr/local/pgsql/data/postmaster.pid');
       read_file       
-----------------------
 61588                +
 /usr/local/pgsql/data+
 1401041744         ...