Hello. PostgreSQL is very fast when we perform (even on a huge table)
ALTER TABLE ... ADD COLUMN ... NULL; (nullable without a default value). This is because of NULL bitmap in tuples. And it's greatest feature for a developer! But another very common-case query like ALTER TABLE ... ADD COLUMN ... BOOLEAN NOT NULL DEFAULT false; or ALTER TABLE ... ADD COLUMN ... INT NOT NULL DEFAULT 0; for a huge table is performed very slow - this is because PostgreSQL have to re-create all tuples assigning the default value to them. If I have a table with 1 billion rows (for example), I have no chance to perform this query at all - too slow. (In most cases NOT NULL DEFAULT xxx fields are BOOLEAN, flags: it is not handy to have 3-way flags.) So, are there plans to optimize such kind of queries? This could be done by many ways: 1. Store the DEFAULT flag directly in NULL BITMAP (add a bit to NULL bitmap not only for NULLable fields, but also for NOT NULL DEFAULT ... fields). 2. Add another bitmap for each tuple (DEFAULT bitmap). Bit value 0 means that there is a real value in a cell, 1 - that the value is default. 3. The same as (1), but always force default value to be 0 (or false or any other values with meaning "zero") and optimize only these cases.
