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Running ALTER TABLE on any table will check if a TOAST table needs to be added. On shared tables, this would previously fail, thus effectively disabling ALTER TABLE for those tables. On (non-shared) system catalogs, on the other hand, it would add a TOAST table, even though we don't really want TOAST tables on some system catalogs. In some cases, it would also fail with an error "AccessExclusiveLock required to add toast table.", depending on what locks the ALTER TABLE actions had already taken. So instead, just ignore attempts to add TOAST tables to such tables, outside of bootstrap mode, pretending they don't need one. This allows running ALTER TABLE on such tables without messing up the TOAST situation. Legitimate uses for ALTER TABLE on system catalogs include setting reloptions (say, fillfactor or autovacuum settings). (All this still requires allow_system_table_mods, which is independent of this.) Discussion: https://www.postgresql.org/message-id/flat/e49f825b-fb25-0bc8-8afc-d5ad895c7975@2ndquadrant.com
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/.