Yandex is one of the largest internet companies in Europe, operating Russia’s most popular search engine and its most visited website. Yandex.Mail is one of the biggest mail services in Russia with audience of 50+ million users weekly and 200+ million users total. All mail metadata (without message bodies) were stored in Oracle databases with a total size of approximately 300 terabytes (without accounting raids and standbys, only data) and total load of 250.000 transactions per second. It took us several years to migrate all these data and load from Oracle to PostgreSQL. In this talk we will speak about our motivation, architecture decisions, migration process and of course about difficulties faced.