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Up through v11 it was sensible to use the "oid" system column as a foreign key column, but since that was removed there's no visible usefulness in making any of the remaining system columns a foreign key. Moreover, since the TupleTableSlot rewrites in v12, such cases actively fail because of implicit assumptions that only user columns appear in foreign keys. The lack of complaints about that seems like good evidence that no one is trying to do it. Hence, rather than trying to repair those assumptions (of which there are at least two, maybe more), let's just forbid the case up front. Per this patch, a system column in either the referenced or referencing side of a foreign key will draw this error; however, putting one in the referenced side would have failed later anyway, since we don't allow unique indexes to be made on system columns. Per bug #17877 from Alexander Lakhin. Back-patch to v12; the case still appears to work in v11, so we shouldn't break it there. Discussion: https://postgr.es/m/17877-4bcc658e33df6de1@postgresql.org
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.