Petite Abeille wrote:
On Oct 8, 2012, at 10:12 PM, Richard Hipp <d...@sqlite.org> wrote:
ALTER TABLE DROP COLUMN requires writing every row of the table.
Could be a 'logical delete' instead. In other words, a dictionary operation
instead of a data one.
http://www.oracle-base.com/articles/8i/dropping-columns.php#LogicalDelete
I don't think you want to follow the syntax of that Oracle example, the "ALTER
TABLE foo SET UNUSED (bar, baz)", especially since that is reversible and it
could mess up other issues such as effectively invalidating constraints.
*All* SQL actions are /supposed/ to be logical, meaning that DROP COLUMN *is* a
"logical delete", no matter if column data is physically removed or just made
inaccessible.
SQLite can add the DROP COLUMN syntax but just implement it so that tables don't
have to be rewritten, but just that the then-superfluous table data is ignored,
and so you still get O(1).
Of course, if that is done, then in order to have predictable performance we'd
also want to add some other statement one can invoke when they want to reclaim
disk space later, which actually goes and rewrites the table, rather than this
just happening "automatically" (though it could also be configured to happen
automatically).
-- Darren Duncan
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