PostgreSQL: How to union 3 tables sorted by date

Question

I have 3 different SQL queries from 3 different unrelated tables (all using LIMIT and ORDER BY).

I would like to merge and sort the results according to the \"date\"

Answer 1

SELECT.....
 UNION ALL 
SELECT....
 UNION ALL
SELECT ...
 ORDER BY date_field;

For the best performance, apply ORDER BY / LIMIT as late as possible, and avoid it in subqueries.