avoid locking postgres read/write as long on alter default

I'm actually not sure if this will work generally, but you can replace:

-- reads and writes block until it is fully rewritten (hours?)
ALTER TABLE items ADD COLUMN last_update timestamptz 

With:

-- select, update, insert, and delete block until the catalog is update (milliseconds)
ALTER TABLE items ADD COLUMN last_update timestamptz;
-- select and insert go through, some updates and deletes block while the table is rewritten
UPDATE items SET last_update = now();

And avoid locking a table for a potentially long time.

The issue proposes changing the PostgreSQL sequence from ALTER TABLE items ADD COLUMN to a separate UPDATE items SET last_update = now(). Start by verifying the locking behavior and whether this applies generally. Done means avoiding a potentially long table lock while preserving the intended default-column behavior.