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The table-rewriting forms of ALTER TABLE are MVCC-unsafe, in much the same way as TRUNCATE, because they replace all rows of the table with newly-made rows with a new xmin. (Ideally, concurrent transactions with old snapshots would continue to see the old table contents, but the data is not there anymore --- and if it were there, it would be inconsistent with the table's updated rowtype, so there would be serious implementation problems to fix.) This was nowhere documented though, and the problem was only documented for TRUNCATE in a note in the TRUNCATE reference page. Create a new "Caveats" section in the MVCC chapter that can be home to this and other limitations on serializable consistency. In passing, fix a mistaken statement that VACUUM and CLUSTER would reclaim space occupied by a dropped column. They don't reconstruct existing tuples so they couldn't do that. Back-patch to all supported branches.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.