Database with “Open Schema” - Good or Bad Idea?

Question

The co-founder of Reddit gave a presentation on issues they had while scaling to millions of users. A summary is available here.

What surprised me is point 3:

Answer 1

This is a data model known as EAV for entity-attribute-value. It has its uses. A prime example is patient test data which is naturally sparse since there are hundreds of thousands of tests which might be run, but typically only a handful are present for a patient. A table with hundreds of thousands of columns is silly, but a table with EAV makes good sense.