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>40.7. Modules and the <CODE
CLASS="FUNCTION"
>unknown</CODE
> Command</A
></H1
><P
> PL/Tcl has support for autoloading Tcl code when used.
It recognizes a special table, <TT
CLASS="LITERAL"
>pltcl_modules</TT
>, which
is presumed to contain modules of Tcl code. If this table
exists, the module <TT
CLASS="LITERAL"
>unknown</TT
> 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 <A
HREF="pltcl-global.html"
>Section 40.4</A
>.)
</P
><P
> While the <TT
CLASS="LITERAL"
>unknown</TT
> module could actually contain any
initialization script you need, it normally defines a Tcl
<CODE
CLASS="FUNCTION"
>unknown</CODE
> procedure that is invoked whenever Tcl does
not recognize an invoked procedure name. <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
>'s standard version
of this procedure tries to find a module in <TT
CLASS="LITERAL"
>pltcl_modules</TT
>
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 <TT
CLASS="LITERAL"
>pltcl_modfuncs</TT
> provides an index of
which functions are defined by which modules, so that the lookup
is reasonably quick.
</P
><P
> The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> distribution includes
support scripts to maintain these tables:
<TT
CLASS="COMMAND"
>pltcl_loadmod</TT
>, <TT
CLASS="COMMAND"
>pltcl_listmod</TT
>,
<TT
CLASS="COMMAND"
>pltcl_delmod</TT
>, as well as source for the standard
<TT
CLASS="LITERAL"
>unknown</TT
> module in <TT
CLASS="FILENAME"
>share/unknown.pltcl</TT
>. This module
must be loaded
into each database initially to support the autoloading mechanism.
</P
><P
> The tables <TT
CLASS="LITERAL"
>pltcl_modules</TT
> and <TT
CLASS="LITERAL"
>pltcl_modfuncs</TT
>
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 <TT
CLASS="LITERAL"
>pltcl_modules</TT
> (and thus,
not attempt to load the <TT
CLASS="LITERAL"
>unknown</TT
> 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.
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