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This oversight led to data corruption in matviews, manifesting as "could not access status of transaction" before our most recent releases, and "found xmin from before relfrozenxid" errors since then. The proximate cause of the problem seems to have been confusion between the task of preserving dropped-column status and the task of preserving frozenxid status. Those are required for distinct sets of relkinds, and the reasoning was entirely undocumented in the source code. In hopes of forestalling future errors of the same kind, try to improve the commentary in this area. In passing, also improve the remarkably unhelpful comments around pg_upgrade's set_frozenxids(). That's not actually buggy AFAICS, but good luck figuring out what it does from the old comments. Per report from Claudio Freire. It appears that bug #14852 from Alexey Ermakov is an earlier report of the same issue, and there may be other cases that we failed to identify at the time. Patch by me based on analysis by Andres Freund. The bug dates back to the introduction of matviews, so back-patch to all supported branches. Discussion: https://postgr.es/m/CAGTBQpbrY9CdRGGhyBZ9yqY4jWaGC85rUF4X+R7d-aim=mBNsw@mail.gmail.com Discussion: https://postgr.es/m/20171013115320.28049.86457@wrigleys.postgresql.org
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.