psql7=# select count(*) from comuni; count ------- 8342 (1 row) psql7=> EXPLAIN select distretto from comuni union select codice_fiscale from comuni; NOTICE: QUERY PLAN: Unique (cost=1767.19..1808.90 rows=1668 width=12) -> Sort (cost=1767.19..1767.19 rows=16684 width=12) -> Append (cost=0.00..464.84 rows=16684 width=12) -> Seq Scan on comuni (cost=0.00..232.42 rows=8342 width=12) -> Seq Scan on comuni (cost=0.00..232.42 rows=8342 width=12) EXPLAIN psql65=> select count(*) from comuni; count ------- 8342 (1 row) psql65=> EXPLAIN select distretto from comuni union select codice_fiscale from comuni; NOTICE: QUERY PLAN: Unique (cost=848.57 rows=0 width=0) -> Sort (cost=848.57 rows=0 width=0) -> Append (cost=848.57 rows=0 width=0) -> Seq Scan on comuni (cost=424.29 rows=8342 width=12) -> Seq Scan on comuni (cost=424.29 rows=8342 width=12) EXPLAIN