How to select id with max date group by category in PostgreSQL?

This is a perfect use-case for DISTINCT ON – a Postgres specific extension of the standard DISTINCT:

SELECT DISTINCT ON (category)
       id  -- , category, date  -- any other column (expression) from the same row
FROM   tbl
ORDER  BY category, date DESC;

Careful with descending sort order. If the column can be NULL, you may want to add NULLS LAST:

• Sort by column ASC, but NULL values first?

DISTINCT ON is simple and fast. Detailed explanation in this related answer:

• Select first row in each GROUP BY group?

For big tables with many rows per category consider an alternative approach:

• Optimize GROUP BY query to retrieve latest row per user
• Optimize groupwise maximum query