[zorga]

That objection doesn't make sense. Queries are nothing more than a tree of predicates, how the backend end uses those predicates is not relevant to the API of specifying the predicates. Things like indexes whether in Mongo or in SQL are implementation details that can easily be hidden and not infect the API. You can interpret the tree of predicates into a SQL where clause or into a Mongo index search.

The OP is correct, your app can speak to an internal API without the underlying database infecting your domain code. That in no way implies you can't take advantage of the best of each database.

[int_19h]

This is all rosy in theory. In practice, the way you write the query matters quite a bit. Often even between different SQL implementations.

And it's not just queries. Transactions often have important semantic differences that will be visible on application layer - again, even between different SQL implementations (e.g. MVCC vs locks).

[zorga]

> In practice, the way you write the query matters quite a bit.

Which is hidden in the query interpreter for said db implementation. Each implementation can break down that abstract query into whatever implementation specific query works best in that database.

There's always some abstract way to represent it that doesn't require vendor specific knowledge nor does it remove the ability to apply vendor specific abilities.

Look, I just don't agree with you, I agree with OP. Db specific stuff should be hidden from the domain layer by an abstract query representation and an abstract transaction representation to be plugged in at a later time.

[int_19h]

Have you ever implemented an ORM? I did.

Stuff like "each implementation can break down that abstract query into whatever implementation specific query works best in that database" is wishful thinking. It's like saying that Java is faster than C++, in theory, because JIT can produce better code. And in theory, it can. In practice, we're not there yet. Same thing with high-level database abstractions - they're all either leaky in subtle ways, or they constrain you to extremely basic operations that can be automatically implemented efficiently on everything (but e.g. forget joins).

[zorga]

> Have you ever implemented an ORM? I did.

Several actually, which is why I know what I'm talking about; I've explored this area extensively. When Fowler first released PEAA I dug and went nuts and spent years coding up and exploring all the possible approaches and figuring out which ones I liked and why and which ones I didn't and why.

> Same thing with high-level database abstractions - they're all either leaky in subtle ways, or they constrain you to extremely basic operations that can be automatically implemented efficiently on everything (but e.g. forget joins).

If you're doing joins in your ORM, frankly, you're doing it wrong. Most ORM's do it wrong, they try and replace what a db does best; the right way to do it is to keep joins in the db. The role of an ORM when used properly is to map tables and views into objects and allow querying over those tables and views with an abstract query syntax. Joins belong in a view, not in code. It's called the object relational impedance mismatch for a reason, you have to draw a line in a reasonable place to get anything reasonable to work well and putting joins into the ORM is crossing that line and is why most ORM's utterly suck. Joins aren't queries, they're projections; put the queries in the code and the projections into the database, this works perfectly and lets each side do what it does best. Queries are easily abstracted, projections are not, projections don't belong in the ORM.

[int_19h]

Any language with named tuples has a type system that is sufficiently expressive to handle joins without any sort of impedance mismatch. So, the only reason to avoid them is exactly the one that I cited earlier - the underlying implementation difference between databases.

[zorga]

> Any language with named tuples has a type system that is sufficiently expressive to handle joins without any sort of impedance mismatch

Incorrect. Named tuples will give you nothing back but a result set; the impedance mismatch refers to the mismatch between result sets and a domain model; getting tuples back doesn't remotely address this problem. I'd suggest you don't understand what the objection relational impedance mismatch problem actually is.

It's not a type system problem, it's fundamental mismatch between the relational paradigm and the object oriented paradigm. If a domain model has customers and addresses, and you do a relational query that joins the customer table and address table to return only the customer name and address city, the resulting set (name, city) doesn't map to the domain objects and isn't enough data for the domain model to load either of those objects which may contain various business rules. This is what the impedance mismatch refers to, relational projections of new result sets simply do not map to the OO way of doing things. Joins that create new projections are a relational concept that have no place in the object oriented world view: objects don't do joins, and object queries don't return differently shaped objects.

Hacks like partial loading of domain objects are attempts to mitigate the impedance mistmatch, but they do not solve it; they cannot solve what is a fundamental difference between two different ways of seeing data. Data is primary in the relational model and its shape can change on a per query basis, this is incompatible with the object oriented view of the world in which whole objects are primary and data is encapsulated and thus hidden.

The object relational impedance mismatch does not refer to a language problem, it refers to a difference in paradigm between OO and relational. It exists in all language regardless of the languages abilities and it's not a problem that can be solved, only mitigated, if you want to use both paradigms. You can solve the problem by avoiding using two paradigms, by either bringing the relational model into the application and not using OO or by using an object database.

[kstrauser]

I don't think there's a point in arguing this further. The other poster seems unwilling to understand that there are differences it databases within a category, let alone that there are multiple categories of databases with entirely different characteristics.