Performing Salaah between pillars

In a Masjid, a row is usually left empty after every few rows, as it has several pillars.

1. If it is left empty, is the Salah of those behind it valid?

2. Is Salah valid between the pillars?

3. Once, people prayed in that row to the right of a pillar, but to the left of the pillar the row was empty. Was the Salah of those who prayed to the right of the pillar valid?