Plesk Service Team is pleased to present you Parallels Plesk Panel Database Scheme Poster.
You can find the detailed structure of Plesk database and use it to eliminate possible database inconsistencies and to better understand internal relationship between objects. Using this scheme requires a good familiarity with the syntax of SQL queries and skills in database administration.
Please share your experience and opinion in corresponding thread on Parallels forum http://forum.parallels.com/showthread.php?t=210218