user: Question: Which Surface has a Tournament of Olbia?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent in the final', 'Score']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1998.0, 'Germany F13', 'Clay', 'Mehdi Tahiri', '6–3, 7–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Tournament" = 'Olbia';