Optimize GROUP BY query to retrieve latest row per user

Question

I have the following log table for user messages (simplified form) in Postgres 9.2:

CREATE TABLE log (
    log_date DATE,
    user_id  INTEGER,
    payload           


        

Answer 1

This is not a standalone answer but rather a comment to @Erwin's answer. For 2a, the lateral join example, the query can be improved by sorting the users table to exploit the locality of the index on log.

SELECT u.user_id, l.log_date, l.payload
  FROM (SELECT user_id FROM users ORDER BY user_id) u,
       LATERAL (SELECT log_date, payload
                  FROM log
                 WHERE user_id = u.user_id -- lateral reference
                   AND log_date <= :mydate
              ORDER BY log_date DESC NULLS LAST
                 LIMIT 1) l;

The rationale is that index lookup is expensive if user_id values are random. By sorting out user_id first, the subsequent lateral join would be like a simple scan on the index of log. Even though both query plans look alike, the running time would differ much especially for large tables.

The cost of the sorting is minimal especially if there is an index on the user_id field.