尽管很早就用PD，但是这个概念一直不清楚。
今天作文档随便查一下，解释如下：
Physical Data Model Basics
What is a PDM?
The PDM is a database design tool for defining the implementation of physical structures and data queries.
Depending on the type of database you want to design, you will use different types of diagrams in the PDM.
Database Diagram
Operational Physical diagram to define the physical implementation of the database
Date warehouse or Data mart Physical diagram to store business data
OLAP Multidimensional diagram to define the possible queries to perform on the operational data
