On 10/5/16, Andres Freund <and...@anarazel.de> wrote: > 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.
I'm sorry, while I was writting "pre-alter" I meant "pre-alter-add-column" era (not "pre-alter-set-default"), all later default changes "current" default, whereas "pre-alter-add-column" adds value if current column number < TupleDesc.natts. All your DDL are in the "post-alter-add-column" era. -- Best regards, Vitaly Burovoy -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
