Re: postgresql vs mysql

Scott Marlowe wrote:
> You can't change a table in any way without rewriting the whole thing,
> resulting in a very long wait and a complete table lock on any alter
> table action on big tables.  Don't forget that if you've got a really
> big table, you need that much space free on the drive to alter the table
> for the rewrite that's going to take place.

Forgive a dumb question: What does postgresql do with ALTER TABLE?
What sort of modifications do not require time proportional to the
number of rows in the table?

Jack Orenstein