DROP column: documentation unclear

Hi,

The documentation about DROP COLUMN is a bit unclear:

| The DROP COLUMN form does not physically remove the column, but simply
| makes it invisible to SQL operations. Subsequent insert and update
| operations in the table will store a null value for the column. Thus,
| dropping a column is quick but it will not immediately reduce the on-disk
| size of your table, as the space occupied by the dropped column is not
| reclaimed. The space will be reclaimed over time as existing rows are
| updated.

"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?

chees
-- vbi

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