sql - Sort by column ASC, but NULL values first?

Question

I need to sort a PostgreSQL table ascending by a date/time field, e.g. last_updated.

But that field is allowed to be empty or null and I want records with null in last_updated come before non-null last_updated.
Is this possible?

order by last_updated asc  -- and null last_updated records first ??

Answer 1

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