Postgres has the NULLS FIRST | LAST modifiers for ORDER BY expression:
... ORDER BY last_updated NULLS FIRST
The typical use case is with descending sort order (DESC), which produces the complete inversion of the default ascending order (ASC) with null values first – which is often not desirable. To sort NULL values last:
... ORDER BY last_updated DESC NULLS LAST
To support the query with an index, make it match:
CREATE INDEX foo_idx ON tbl (last_updated DESC NULLS LAST);
Postgres can read btree indexes backwards, but for some query plans it matters where NULL values are appended. See:
- Performance impact of view on aggregate function vs result set limiting