Re: DROP column: documentation unclear

In article <20100308213549.GB660@svana.org>,
Martijn van Oosterhout <kleptog@svana.org> writes:

>> "subsequent ... will store a null value" would imply that deleted columns
>> will still take some place, while "the space will be reclaimed ..." would
>> suggest that new rows (insert or updates in mvcc) don't have the deleted
>> column anymore - I'm not quite sure how to interpret this.  What is pg
>> doing?

> What you're missing is that in postgres NULLs are stored as a bit in
> the header and there is no data. So in a sense NULLs take no space
> (well, one bit) which means both statements are true.

But if you already have eight nullable columns, the (maybe originally
non-null) column which has been dropped would cause the header to be
one byte larger, wouldn't it?