Difference between physical model creation from a conceptual model and from a logical model

[Former Member]

As per the power designer flow conceptual model -> logical model -> physical model !!

When we are creating conceptual model under tool there is a option to generate direct physical model with out creating a logical model.

Is there any difference between creation of physical model from "conceptual-->logical-->physical" and "conceptual--> physical"

[former_member192453 · Accepted solution]

In my opinion, the short answer is no. If you are using the same CDM, then generating CDM -> PDM or CDM ->LDM ->PDM should result in essentially, the same PDM.

Within PowerDesigner, you can start with any model type. There is nothing in the tool that forces you to follow a specific flow of conceptual model -> logical model -> physical model. When considering a CDM or a LDM, I look at the differences between the model types.

For example:

In a CDM, you can define data items and entity attributes. In a LDM, you can only define entity attributes.

In the CDM, the foreign attribute migration does not occur until you generate a LDM or PDM. In the LDM, the foreign attribute migrates immediately.

In the LDM, you can allow inherited attributes to diverge. In the CDM, this option does not exist.

Whether to start with a CDM or LDM is a personal choice.

HTH,

Anthony

[Former Member]

If you're following CDM -> PDM path, then you'll end up with doing following things in PDM :-

1. Role Naming your foreign keys, updating their definitions based on their role name because foreign keys doesn't migrate to child entity.

2. Alternate Keys (Business Keys)/ Indexes. If a foreign key is part of alternate key then you will not be able to define AKs in CDM model and you'll end up doing same it in PDM.

3. CDM's data items functionality can be replaced by 'Replicate attributes' in LDM which I think is better than data items as if you replicate an attribute in Entity B from Entity A, then you need to go to Entity A to make any changes for that attribute if replicate attribute properties are tightly coupled. However, data items (CDM) can be changed through any entity in CDM model.

4. Column ordering : As foreign keys doesn't migrate in CDM and default template is put foreign keys after Primary key and if your organization follows a standard to put audit columns after primary keys, then you'll end up reordering your columns in PDM.

You can skip CDM if possible and start with LDM. Or just use CDM for conceptual modeling and generate LDM from it. I prefer to make very less changes in PDM model until it's related to a database implementation.

As Anthony said, whether to start with a CDM or LDM is a personal choice.

Cheers !

Arvin