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by HelloNurse 2219 days ago
If adding a nullable column is free, it probably means that the DBMS is able to distinguish multiple layouts for the same table: existing rows in which the new column doesn't actually exist and is treated as NULL, and newly written rows in which there is space for the new column.

But dropping a column is different: even if the DBMS performs a similar smart trick (ignoring the value of the dropped column that is contained in old rows) space is still wasted, and it can only be reclaimed by rewriting old files.
1 comments
calpaterson 2219 days ago
Dropping a column in postgres is also instant, so yes, it uses the same trick.

Deleting a row is similar too - the row is not removed from the heap page and the database does not get smaller (though if that page gets rewritten the row is not kept). Last time I used innodb it didn't actually return free heap pages to the filesystem at all so no matter how much you deleted the database file never got smaller.

EDIT: Looks like that's still the case now: https://bugs.mysql.com/bug.php?id=1341

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