How to Order a PostgreSQL Table by Date/Time, Prioritizing NULL Values?

PostgreSQL table sorted by date/time, handling NULL values ​​first

Database administrators often need to sort data in a specific order, and PostgreSQL provides a variety of options to customize the sorting behavior. A common scenario is to sort by date/time fields while handling NULL values ​​according to the desired order.

Problem: NULL value prioritization

Suppose you need to sort the table in ascending order by the 'last_updated' date/time field, and process records with NULL values ​​first. The challenge is that PostgreSQL's default ascending order (ASC) puts NULL values ​​at the end of the result.

Solution: Use NULLS FIRST modifier

PostgreSQL provides the 'NULLS FIRST | LAST' modifier for ORDER BY expressions. Adding the 'NULLS FIRST' modifier to the 'last_updated' field in the ORDER BY clause ensures that records with NULL values ​​appear before non-NULL values:

... ORDER BY last_updated NULLS FIRST

NULLS LAST and index support

For descending sorting (DESC), a typical use case is 'NULLS LAST', which reverses the default ascending order, placing NULL values ​​at the beginning. To sort NULL values ​​last, use:

... ORDER BY last_updated DESC NULLS LAST

To optimize queries with indexes, make sure the index matches the sort order:

CREATE INDEX foo_idx ON tbl (last_updated DESC NULLS LAST);

Performance Notes

The position of NULL values ​​affects query performance. For some query plans, PostgreSQL attempts to append a NULL value to the beginning ('NULLS FIRST') or the end ('NULLS LAST'). The following link provides insights into the performance impact:

• Performance impact of aggregate function view on result set limits: https://www.php.cn/link/ff3d65f00548c4e0c189bb01825f832e

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