Different results when specifying DEFAULT values through ALTER TABLE

Hi,

I observed the following on PostgreSQL 8.1.3 (on Windows 2000).

(1) alter table invoice add column active boolean default true;

This will add a new column "active" with a default clause as "true" and also sets the value "true" to all existing rows
ofthe table. 

BUT

(2) ALTER TABLE invoice  ADD COLUMN active boolean;
ALTER TABLE invoice  ALTER COLUMN active SET DEFAULT true;

Will result in only adding the column "active" with default clause as "true". It will NOT set the value "true" for
existingrows. 

Is there a particular reason for this difference in behaviour? (Or am I missing something in the syntax?)

Thanks and regards,

-Thusitha Kodikara
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