1. How to get the name of user currently connected to the engine?
MSSQL: SELECT SYSTEM_USER
Oracle: select user from dual
2. How to list schemas/databases available to user currently connected to the engine?
Oracle: SELECT * FROM ALL_USERS
3. How to retrieve the size of selected schema/database?
MSSQL: look at the column [db_size] returned by sp_helpdb
Oracle: select Sum(bytes) from dba_segments where owner=SCHEMANAME group by tablespace_name,owner