Which is faster: Many rows or many columns?

Question

In MySQL, is it generally faster/more efficient/scalable to return 100 rows with 3 columns, or 1 row with 100 columns?

In other words, when storing many key => value pairs related to a record, is it better to store each key => value pair in a separate row with with the record_id as a key, or to have one row per record_id with a column for each key?

Also, assume also that keys will need to be added/removed fairly regularly, which I assume would affect the long term maintainability of the many column approach once the table gets sufficiently large.

Edit: to clarify, by "a regular basis" I mean the addition or removal of a key once a month or so.

Answer 1

You should never add or remove columns on a regular basis.

Answer 2

http://en.wikipedia.org/wiki/Entity-Attribute-Value_model

There are a lot of bad things about this model and I would not use it if there was any other alternative. If you don't know the majority (except a few user customizable fields) of data columns you need for your application, then you need to spend more time in design and figure it out.