chore: Use lazy rather than joined eager loading

[john-bodley]

Description

#1361 introduced Joined Eager Loading for joining one-to-many or many-to-many model relationships, however per the SQLAlchemy example this results in a slew of LEFT OUTER JOINs which will rapidly result into a expansive interim result set—from a multiplicative combinatorial sense—when a model has multiple high cardinality relationships. Furthermore the result set—which is repetitive in nature—requires further overhead by SQLAlchemy to de-duplicate the rows against the actual model record.

Given that the FAB response is an addition of relationships rather than a multiplication of relationships it seems like Lazy Loading is more desirable. Granted this results in one query per relationship, but the interim result sets are typically much smaller non-repetitive in nature (especially for high cardinality relationships) and thus more performant.

For example in Superset at Airbnb we have a virtual dataset which is comprised of a thousands of metrics and columns. Previously a query to obtain the dataset with the associated metric and column IDs would time out (the join would result in an interim result set of 100M+ rows) whereas with lazy loading the query takes << 5 seconds.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[dpgaspar]

@john-bodley thanks!

I'll check possible implications on other endpoints, maybe we can make this has an option set at the REST model view or model itself

[john-bodley]

@dpgaspar I wonder rather than having FAB explicitly define the relationship loading technique it should defer to the loading technique defined by the model. This should ensure the loading is optimized based on the explicit implementation/use case.

This can be achieved via the default loader and having the models specify—the via the lazy parameter of the ORM relationship (which defaults to select)—the relationship loader, i.e., in the case of Superset's SqlaTable model this would be:

class SqlaTable(Model):
    columns = relationship(
        "TableColumn",
        lazy="selectin",
    )

If you're in agreement with this approach I'll close this PR in favor of new PR which uses default rather than joined (eager) loading.

[john-bodley]

Closing in favor of #1961.