"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.
To force immediate reclamation of space occupied by a dropped column, you can execute one of the forms of ALTER TABLE that performs a rewrite of the whole table. This results in reconstructing each row with the dropped column replaced by a null value. "
Thank you so much. When you said "you can execute one of the forms of ALTER TABLE that performs a rewrite
of the whole table." Does it mean that post "alter table drop column" the vacuum is going to run longer as it will try to clean up all the rows and recreate the new rows? But then how can this be avoidable or made better without impacting the system performance
"Impact" is a non-specific word. "How much impact" depends on how many autovacuum workers you've set it to use, and how many threads you set in vacuumdb.
and blocking others?
VACUUM never blocks.
Anyway, DROP is the easy part; it's ADD COLUMN that can take a lot of time (depending on whether or not you populate the column with a default value).
I'd detach all the partitions from the parent table, and then add the new column to the not-children in multiple threads, add the column to the parent and then reattach all of the children. That's the fastest method, though takes some time to set up.