Come raggruppare per settimana in postgresql

Se hai più anni, dovresti prendere in considerazione anche l'anno. Un modo è:

SELECT date_part('year', author_date::date) as year,
       date_part('week', author_date::date) AS weekly,
       COUNT(author_email)           
FROM commits
GROUP BY year, weekly
ORDER BY year, weekly;

Un modo più naturale per scriverlo usa date_trunc() :

SELECT date_trunc('week', author_date::date) AS weekly,
       COUNT(author_email)           
FROM commits
GROUP BY weekly
ORDER BY weekly;