How to check total datafile size created per month in Oracle

Rollup datafile creation_time by year and month. Rough proxy for when tablespaces were expanded — spikes here often line up with data growth events, migrations, or restore points.

SELECT TO_CHAR(creation_time, 'RRRR') AS year,
       TO_CHAR(creation_time, 'MM')   AS month,
       ROUND(SUM(bytes) / 1024 / 1024 / 1024) AS gb
FROM   v$datafile
GROUP BY TO_CHAR(creation_time, 'RRRR'),
         TO_CHAR(creation_time, 'MM')
ORDER BY 1, 2;