On 2016-10-05 15:23:05 -0700, Vitaly Burovoy wrote: > On 10/5/16, Andres Freund <and...@anarazel.de> wrote: > > On 2016-10-05 11:58:33 -0700, Serge Rielau wrote: > >> Dear Hackers, > >> I’m working on a patch that expands PG’s ability to add columns to a table > >> without a table rewrite (i.e. at O(1) cost) from the > >> nullable-without-default to a more general case. > > > > If I understand this proposal correctly, altering a column default will > > still have trigger a rewrite unless there's previous default? > > No, "a second “exist default"" was mentioned, i.e. it is an additional > column in a system table (pg_attribute) as default column values of > the "pre-alter" era. It solves changing of the default expression of > the same column later.
Don't think that actually solves the issue. The default might be unset for a while, for example. Essentially you'd need to be able to associate arbitrary number of default values with an arbitrary set of rows. ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT; INSERT id = 1; ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 1; ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT; INSERT id = 2; ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 2; ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT; INSERT id = 3; ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 3; The result here would be that there's three rows with a default value for foo that's the same as their id. None of them has that column present in the row. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
