39.7. Modules and the unknown Command

PL/Tcl has support for autoloading Tcl code when used. It recognizes a special table, pltcl_modules, which is presumed to contain modules of Tcl code. If this table exists, the module unknown is fetched from the table and loaded into the Tcl interpreter immediately before the first execution of a PL/Tcl function in a database session. (This happens separately for each Tcl interpreter, if more than one is used in a session; see Section 39.4.)

While the unknown module could actually contain any initialization script you need, it normally defines a Tcl unknown procedure that is invoked whenever Tcl does not recognize an invoked procedure name. PL/Tcl's standard version of this procedure tries to find a module in pltcl_modules that will define the required procedure. If one is found, it is loaded into the interpreter, and then execution is allowed to proceed with the originally attempted procedure call. A secondary table pltcl_modfuncs provides an index of which functions are defined by which modules, so that the lookup is reasonably quick.

The Postgres-XC distribution includes support scripts to maintain these tables: pltcl_loadmod, pltcl_listmod, pltcl_delmod, as well as source for the standard unknown module in share/unknown.pltcl. This module must be loaded into each database initially to support the autoloading mechanism.

The tables pltcl_modules and pltcl_modfuncs must be readable by all, but it is wise to make them owned and writable only by the database administrator. As a security precaution, PL/Tcl will ignore pltcl_modules (and thus, not attempt to load the unknown module) unless it is owned by a superuser. But update privileges on this table can be granted to other users, if you trust them sufficiently.