Behaviour adding a column with and without a default (prior to PG11)

Hi,

The documentation (https://www.postgresql.org/docs/10/ddl-alter.html) clearly states that adding a column with a default requires updating all the rows in the table, to store the new column value (this is prior to PG11, obviously).

I can understand why this behaviour was required when adding a not null column, but for a nullable column, it also seems reasonable to me that existing rows would not be updated and the default added to apply to new rows only.

Obviously the desired effect of adding the column default without rewriting all the rows in the table can be achieved by adding the nullable column first with no default, then changing the default, but out of curiosity; was there a specific technical reason for the behaviour described above, or was it a conscious design choice?

Many thanks,

Joe.

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