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It's possible to use a CHECK (col IS NOT NULL) constraint to skip scanning a table for nulls when adding a NOT NULL constraint on the same column. However, if the CHECK constraint is dropped on the same command that the NOT NULL is added, this fails, i.e., makes the NOT NULL addition slow. The best we can do about it at this stage is to document this so that users aren't taken by surprise. (In Postgres 18 you can directly add the NOT NULL constraint as NOT VALID instead, so there's no longer much use for the CHECK constraint, therefore no point in building mechanism to support the case better.) Reported-by: Andrew <psy2000usa@yahoo.com> Reviewed-by: David G. Johnston <david.g.johnston@gmail.com> Discussion: https://postgr.es/m/175385113607.786.16774570234342968908@wrigleys.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.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/18/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/18/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.