PL/Tcl offers most of the capabilities a function writer has in the C language, with a few restrictions, and with the addition of the powerful string processing libraries that are available for Tcl.
    One compelling good restriction is that
    everything is executed from within the safety of the context of a
    Tcl interpreter.  In addition to the limited command set of safe
    Tcl, only a few commands are available to access the database via
    SPI and to raise messages via elog().  PL/Tcl
    provides no way to access internals of the database server or to
    gain OS-level access under the permissions of the
    PostgreSQL server process, as a C
    function can do.  Thus, unprivileged database users can be trusted
    to use this language; it does not give them unlimited authority.
   
The other notable implementation restriction is that Tcl functions cannot be used to create input/output functions for new data types.
    Sometimes it is desirable to write Tcl functions that are not restricted
    to safe Tcl.  For example, one might want a Tcl function that sends
    email.  To handle these cases, there is a variant of PL/Tcl called PL/TclU
    (for untrusted Tcl).  This is exactly the same language except that a full
    Tcl interpreter is used.  If PL/TclU is used, it must be
    installed as an untrusted procedural language so that only
    database superusers can create functions in it.  The writer of a PL/TclU
    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.
   
    The shared object code for the PL/Tcl and
    PL/TclU call handlers is automatically built and
    installed in the PostgreSQL library
    directory if Tcl support is specified in the configuration step of
    the installation procedure.  To install PL/Tcl
    and/or PL/TclU in a particular database, use the
    CREATE EXTENSION command, for example
    CREATE EXTENSION pltcl or
    CREATE EXTENSION pltclu.