When PG11 added the ability for ALTER TABLE ADD COLUMN to set a constant default value without rewriting the table the doc changes did not note how the new feature interplayed with ADD COLUMN DEFAULT NOT NULL. Previously such a new column required a verification table scan to ensure no values were null. That scan happens under an exclusive lock on the table, so it can have a meaningful impact on database "accessible uptime".
I've attached a patch to document that the new mechanism also precludes that scan. Thanks, James Coleman
v1-0001-Document-atthasmissing-default-avoids-verificatio.patch
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