Methods of data transfer from the Oracle environment to the PostgreSQL environment with stageby- stage organization of actions and determination of the sequence of migration are considered. The main directions for the transformation of the structure of databases, data formats and executed objects are identified. Methods for converting stored procedures and triggers written in procedural languages (PL/SQL and PL/pgSQL) are specified. Software change on nodes directly connected (associated) with the database server and minimization of downtimes during migration are considered, which allows the transfer to be performed for geographically distributed and round-the-clock functioning objects. Examples of changing the speed of query execution before and after migration are given for systems of various types.