but, using the SQL API interface looks interesting...
where can I get more infos about the EA repository schema?
I haven't found anything about... no ER diagrams no detailed documentation about the internal structure of the repositoy...
for example... does the api support JOIN operations?
(I'd like to build a query directy to the DiagramObject... this should solve the problem... )
You can't since there isn't any documentation

Luckily the database is pretty simple.
Diagrams can be found in t_diagram, DiagramObjects in t_diagramObjects and elements in t_Object,...
Depending on the database you use (eap files are MS Access databases) you'll have to adjust the query.
The query you pass to the operation SQLQuery is directly passed on to the database, so if it works on the database it'll work using the API.
Geert
PS. The performance difference between traversing a model, and using the SQLQuery to find the objects you need is pretty huge.
As an example: I created an excel export that had to export about 6000 objects all contained in one rootpackage.
Using the conventional API objects to visit all packages, subpackages etc... took about three hours.
After refactoring to use the SQLQuery operation it took 3 seconds

, including excel document creation and save...