Guido Neitzer <guido.neitzer@xxxxxxxxxxxxx> writes: > What is the technical difference between adding a column to a table > and then apply a "set value = ..." to all columns and adding a column > with a default value = ...? "ADD COLUMN DEFAULT ..." is implemented via a full-table rewrite, so you end up with a version of the table that has no dead space. Unfortunately this requires an exclusive table lock while the rewrite happens, so you lock out other processes from the table for a considerably longer period of time than the UPDATE approach. IIRC it's also not completely MVCC-safe --- committed-dead rows will get removed even if there are old open transactions that should still see those rows as current. Bottom line: there's no free lunch. regards, tom lane
