9b9c5f279e
The behavior of something like ALTER TABLE transactions ADD COLUMN status varchar(30) DEFAULT 'old', ALTER COLUMN status SET default 'current'; is to fill existing table rows with 'old', not 'current'. That's intentional and desirable for a couple of reasons: * It makes the behavior the same whether you merge the sub-commands into one ALTER command or give them separately; * If we applied the new default while filling the table, there would be no way to get the existing behavior in one SQL command. The same reasoning applies in cases that add a column and then manipulate its GENERATED/IDENTITY status in a second sub-command, since the generation expression is really just a kind of default. However, that wasn't very obvious (at least not to me; earlier in the referenced discussion thread I'd thought it was a bug to be fixed). And it certainly wasn't documented. Hence, add documentation, code comments, and a test case to clarify that this behavior is all intentional. In passing, adjust ATExecAddColumn's defaults-related relkind check so that it matches up exactly with ATRewriteTables, instead of being effectively (though not literally) the negated inverse condition. The reasoning can be explained a lot more concisely that way, too (not to mention that the comment now matches the code, which it did not before). Discussion: https://postgr.es/m/10365.1558909428@sss.pgh.pa.us |
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README
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/.