Thursday, 14 March 2019

Abstraction with Database Views

After being away from the relational database world for a few years it’s been interesting coming back and working on a mature system with plenty of SQL code. It’s been said that SQL is the assembly language of databases and when SQL code is written only using its primitives (types and tables) it’s easy to see why.

Way back in 2011 I wrote “The Public Interface of a Database” which was a distillation of my thoughts at the time about what I felt was generally wrong with much of the database code I saw. One aspect in particular which I felt was sorely underutilised was the use of views to build a logical model over the top of the physical model to allow a more emergent design to unfold. This post documents some of the ways I’ve found views to be beneficial in supporting a more agile approach to database design.

Views for Code Reuse

The first thing that struck me about the recent SQL code I saw was how much there was of it. Most queries were pretty verbose and as a consequence you had to work hard to comprehend what was going on. Just as you see the same tired examples around Orders => OrderItems => Products so the code had a similar set of 3 table joins over and over again as they formed the basis for so many queries.

One of the primary uses for database views is as a code reuse mechanism. Instead of copy-and-pasting the same bunch of joins everywhere:

FROM Orders o
INNER JOIN OrderItems oi
ON o.Id = oi.OrderId 
INNER JOIN Products p
ON oi.ProductId = p.Id

we could simply say:

FROM OrdersOrderItemsProducts

This one simplification reduces a lot of complexity and means that wherever we see that name we instantly recognise it without mentally working through the joins in our head. Views are composable too meaning that we can implement one view in terms of another rather than starting from scratch every time.

Naming

However, if the name OrdersOrderItemsProducts makes you wince then I don’t blame you because it’s jarring due to its length and unnaturalness. It’s a classic attempt at naming based on how it’s implemented rather than what it means.

I suspect a difficulty in naming views is part of the reason for their lack of use in some cases. For our classic example above I would probably go with OrderedProducts or ProductsOrdered. The latter is probably preferable as the point of focus is the Products “set” with the use of Orders being a means to qualify which products we’re interested in, like “users online”. Of course one could just easily say “unread messages” and therefore we quickly remember why naming is one of the two hardest problems in computer science.

Either way it’s important that we do spend the time required to name our views appropriately as they become the foundation on which we base many of our other queries.

Views for Encapsulation

Using views as a code reuse mechanism is definitely highly beneficial but where I think they start to provide more value are as a mechanism for revealing new, derived sets of data. The name ProductsOrdered is not radically different from the more long-winded OrdersOrderItemsProducts and therefore it still heavily reflects the physical relationship of the underlying tables.

Now imagine a cinema ticketing system where you have two core relationships: Venue => Screen => SeatingPlan and Film => Screening => Ticket => Seat. By navigating these two relationships it is possible to determine the occupancy of the venue, screen, showing, etc. and yet the term Occupancy says nothing about how that is achieved. In essence we have revealed a new abstraction (Occupancy) which can be independently queried and therefore elevates our thinking to a higher plane instead of getting bogged down in the lengthy chain of joins across a variety of base tables.

Views for Addressing Uncertainty

We can also turn this thinking upside down, so that rather than creating something new by hiding the underlying existing structure, we can start with something concrete and re-organise how things work underneath. This is the essence of refactoring – changing the design without changing the behaviour.

When databases were used as a point of integration this idea of hiding the underlying schema from “consumers” made sense as it gave you more room to change the schema without breaking a bunch of queries your consumers had already created. But even if you have sole control over your schema there is still a good reason why you might want to hide the schema, nay implementation, even from much of your own code.

Imagine you are developing a system where you need to keep daily versions of your customer’s details easily accessible because you regularly perform computations across multiple dates [1] and you need to use the correct version of each customer’s data for the relevant date. When you start out you may not know what the most appropriate way to store them because you do not know how frequently they change, what kinds of changes are made, or how the data will be used in practice.

If you assume that most attributes change most days you may well plump to just store them daily, in full, e.g.

| Date       | Name      | Valuation | ... | 
| 2019-03-01 | Company A | £102m     | ... |  
| 2019-03-01 | Company B | £47m      | ... |  
| 2019-03-02 | Company A | £105m     | ... |  
| 2019-03-02 | Company B | £42m      | ... |  
| 2019-03-03 | Company A | £105m     | ... |  
| 2019-03-03 | Company B | £42m      | ... |

On the contrary, if the attributes rarely change each day then maybe we can version the data instead:

| Name      | Version | Valuation | ... |
| Company A | 1       | £147m     | ... |
| Company A | 2       | £156m     | ... |
| Company B | 1       | £27m      | ... |

So far so good, but how do we track which version belongs to which date? Once again I can think of two obvious choices. The first is much like the original verbose table and we record it on a daily basis:

| Date       | Name      | Version |
| 2019-03-01 | Company A | 1       |
| 2019-03-01 | Company B | 1       |
| 2019-03-02 | Company A | 1       |
| 2019-03-02 | Company B | 2       |

The second is to coalesce dates with the same version creating a much more compact form:

| From       | To         | Name      | Version |
| 2019-03-01 | (null)     | Company A | 1       |
| 2019-03-01 | 2019-03-01 | Company B | 1       |
| 2019-03-02 | (null)     | Company B | 2       |

Notice how we have yet another design choice to make here – whether to use NULL to represent “the future”, or whether to put today’s date as the upper bound and bump it on a daily basis [2].

So, with all those choices how do we make a decision? What if we don’t need to make a decision, now? What if we Use Uncertainty as a Driver and create a design that is easily changeable when we know more about the shape of the data and how it’s used?

What we do know is that we need to process customer data on a per-date basis, therefore, instead of starting with a Customer table we start with a Customer view which has the shape we’re interested in:

| Date | Name | Valuation | ... | 

We can happily use this view wherever we like knowing that the underlying structure could change without us needing to fix up lots of code. Naturally some code will be dependent on the physical structure, but the point is that we’ve kept it to a bare minimum. If we need to transition from one design to another, but can’t take the downtime to rewrite all the data up-front, that can often be hidden behind the view too.

Views as Interfaces

It’s probably my background [3] but I can’t help but notice a strong parallel in the latter two examples with the use of interfaces in object-oriented code. George Box reminds us that “all models are wrong, but some are useful” and so we should be careful not to strain the analogy too far but I think there is some value in considering the relationship between views and tables as somewhat akin to interfaces and classes, at least for the purposes of encapsulation as described above.

On a similar note we often strive to create and use the narrowest interface that solves our problem and that should be no different in the database world either. Creating narrower interfaces (views) allows us to remain more in control of our implementation by leaking less.

One final type related comparison that I think worthy of mention is that it’s easier to spot structural problems when you have a “richer type system”, i.e. many well-named views. For example, if a query joins through ProductsOrdered to get to UserPreferences you can easily see something funky is going on.

Embracing Change

When you work alongside a database where the SQL code and schema gets refactored almost as heavily as the services that depend on it is a pleasurable experience [4]. Scott Ambler wrote a couple of books over a decade ago (Refactoring Databases: Evolutionary Database Design and Agile Database Techniques) which convinced me long ago that it was possible to design databases that could embrace change. Making judicious use of views certainly helped achieve that in part by keeping the accidental complexity down.

Admittedly performance concerns, still a dark art in the world of databases, gets in the way every now and but I’d rather try to make the database a better place for my successors rather than assume it can’t be done.

 

[1] In investment banking it’s common to re-evaluate trades and portfolios on historical dates both for regulatory and analytical purposes.

[2] Some interesting scenarios crop up here when repeatability matters and you have an unreliable upstream data source.

[3] I’m largely a self-taught, back-end developer with many years of writing C++ and C# based services.

[4] Having a large suite of database unit tests, also written in T-SQL, really helped as we could use TDD on the database schema too.

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