20.1. Overview

PL/Tcl offers most of the capabilities a function writer has in the C language, except for some restrictions.

The good restriction is that everything is executed in a safe 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(). There is no way to access internals of the database backend or to gain OS-level access under the permissions of the PostgreSQL user ID, as a C function can do. Thus, any unprivileged database user may be permitted to use this language.

The other, implementation restriction is that Tcl procedures 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 mail. To handle these cases, there is a variant of PL/Tcl called PL/TclU (for untrusted Tcl). This is the exact 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 for the PL/Tcl and PL/TclU call handlers is automatically built and installed in the PostgreSQL library directory if Tcl/Tk 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 createlang script, for example createlang pltcl dbname or createlang pltclu dbname.