On Thu, May 19, 2016 at 7:33 PM, Jeff Janes <jeff.ja...@gmail.com> wrote: > I recently had to run something like: > > alter table pgbench_accounts add locked text check (locked != 'unlocked'); > > And was surprised that it took several minutes to complete as it > scanned the whole table. > > The new column is going to start out as NULL in every row, so there is > no need to validate the check constraint by reading the table as it > can be proven from first principles. Correct?
Right. If there were a DEFAULT on the new column that would of course be different, and you can also do thinks like CHECK (a != b) here. However, if the CHECK constraint does not reference any column other than the newly-added one, and if the new column will have the same value for every row either because there is no default or because the default is a constant, then we can test the CHECK constraint just once against the value that all new rows will have instead of testing it once per row. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers