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Commit 866566a690bb9916 is insufficient to prevent dump/reload failures when using transform modules in a database with both plpython2 and plpython3 installed. The reason is that the transform extension scripts use DO blocks as a mechanism to pull in the libpython library before creating the transform function. It's necessary to preload the library because the dynamic loader won't do it for us on every platform, leading to "unresolved symbol" failures when the transform library is loaded. But it's *not* necessary to execute Python code, and doing so will provoke a multiple-Pythons-are-loaded error even after the preceding commit. To fix, use LOAD instead of a DO block. That requires superuser privilege, but creation of a C function does anyway. It also embeds knowledge of the underlying library name for each PL language; but that's wired into the initdb-time contents of pg_pltemplate too, so that doesn't seem like a large problem either. Note that CREATE TRANSFORM as such doesn't call the language module at all. Per a report from Paul Jones. Back-patch to 9.5 where transform modules were introduced.
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.