Business logic is supposed to be the part of the application where you deal with customer or user facing decisions and computations. It is often argued that this part should be well separated from the rest of the technical infrastructure of your code. Of course, SQL and relational database design is meant to support your business cases (or user stories), so then we can ask ourselves if SQL should be part of your business logic implementation. Or actually, how much of your business logic should be SQL?

As the database model is meant to support your business activity as a whole, it’s quite easy to see that the SQL schema you are working with already implements an important layer of your business logic. Its goal is to implement as much as possible of it, so that when you use the accounting, the back office and the user facing applications on top of it, they all share the same understanding of the database and respect the same set of validity constraints and consistency rules.

My argument then is that every SQL query you send to the server embeds some business logic. Even those you didn’t write because you’re using that awesome ORM tool… but what does your ORM knows exactly about your business case, or your user stories?

A very simple example

In the following example, we are going to first define a business case we want to implement, and then have a look at the SQL statement that we would be using to solve it. We are using the Chinook database again, it models a music collection of tracks, artists, albums, and genre.

With the latest pgloader code (yet to be released at the time of this writing), you can install the Chinook database in a single command: $ createdb chinook $ pgloader https://github.com/lerocha/chinook-database/raw/master/ChinookDatabase/DataSources/Chinook_Sqlite_AutoIncrementPKs.sqlite pgsql:///chinook Well ok a single command line once you’ve created the target database. I guess that makes it two commands.

Let’s pick quite a simple user story: display the list of albums from a given artist, each with its total duration.

In the Chinook model we have a per-track duration field, named milliseconds. Each track is associated with an album through its albumid, and each album is the work of one artist that we reach through artistid. Let’s write a query for solving our business case:

select album.title as album, sum (milliseconds) * interval '1 ms' as duration from album join artist using (artistid) left join track using (albumid) where artist.name = 'Red Hot Chili Peppers' group by album order by album;

The output is:

album │ duration ═══════════════════════╪══════════════════════════════ Blood Sugar Sex Magik │ @ 1 hour 13 mins 57.073 secs By The Way │ @ 1 hour 8 mins 49.951 secs Californication │ @ 56 mins 25.461 secs (3 rows)

What we see here is a direct translation from the business case (or user story if you prefer that term) into a SQL query. The SQL implementation uses joins and computations that are specific to both the data model and the use case we are solving.

Business Logic in the Application Code

Now, we could decide that the application’s code is where to implement our business case, because that’s easier to maintain in the long run. I wrote a direct implementation of the Chinook model in Python and then wrote the same query against the Python model.

The goal of this Python exercise is to obtain classic application’s code and to mimic to some degree the usual layers of abstractions such as found in ORM librairies. Here goes:

#! /usr/bin/env python3 # -*- coding: utf-8 -*- import psycopg2 import psycopg2.extras import sys from datetime import timedelta DEBUGSQL = False PGCONNSTRING = "dbname=chinook application_name=cdstore" class Model ( object ): tablename = None columns = None @classmethod def buildsql (cls, pgconn, **kwargs): ... @classmethod def fetchone (cls, pgconn, **kwargs): ... @classmethod def fetchall (cls, pgconn, **kwargs): ... class Artist (Model): tablename = "artist" columns = [ "artistid" , "name" ] ... class Album (Model): tablename = "album" columns = [ "albumid" , "title" ] ... class Track (Model): tablename = "track" columns = [ "trackid" , "name" , "milliseconds" , "bytes" , "unitprice" ] ... if __name__ == '__main__' : if len (sys.argv) > 1 : pgconn = psycopg2.connect(PGCONNSTRING) artist = Artist.fetchone(pgconn, name=sys.argv[ 1 ]) for album in Album.fetchall(pgconn, artistid=artist. id ): ms = 0 for track in Track.fetchall(pgconn, albumid=album. id ): ms += track.duration duration = timedelta(milliseconds=ms) print ( " %25s : %s " % (album.title, duration)) else : print ( 'albums.py <artist name>' )

When we run the code we have the same result as before with the query:

$ ./albums.py "Red Hot Chili Peppers" Blood Sugar Sex Magik: 1 :13:57.073000 By The Way: 1 :08:49.951000 Californication: 0 :56:25.461000

What can we learn about those two implementations of the same business case?

Correctness

In the application’s code implementation we are doing 5 queries instead of one. What happens if an album had been badly assigned to our artist and is reassigned while our program runs? Well we might fetch the album, but then fail to find any track associated with it in the next query thus report a zero duration.

In this business case it might not be very costly, well except if you charge by album’s duration in some kind of library access and you’re stuck in the 90s for some reason.

What I mean is that it’s easy to transpose this example to your own business and see if you are subject to correctness issues in your implementation. Doing several queries without setting a proper Transaction Isolation Level will undoubtly open the door of inconsistencies in the information your application retrieves. Maybe for billing purposes.

Efficiency

Another problem with the application’s code implementation as written is with its very bad efficiency. As this artists has 3 albums in our data set, we are doing 5 queries here. In my test environement the query runs in about 2ms on the server… laptop really. The network ping in a real setup is usually around 1ms or 2ms, so when going from a single 2ms query to 5 simpler queries you are actually adding 4 round-trips to your application.

So for the same result, 5 queries, say 1ms each, with a very good 1ms round-trip, that’s already 10ms instead of 3ms in the single-query case.

Also the Python code now needs to retrieve way more information than needed and will store and scan that in the local memory. So we are consuming a disproportionate amount of memory, network latency and network bandwidth compared to the SQL only solution. Our PostgreSQL server still has to fetch the same amount of data in its backend’s memory, and scan the tracks milliseconds to compute each album’s duration. Good chances are that those data (if considered hot in your application) are already cached in memory tho.

And this is not the only problem. In terms of scalability of your application, running 5 times as many queries might not turn cheap. Maybe you don’t need this size of a caching layer in front of your API servers after all…

Maintenance

In terms of code maitenance we have to compare the SQL with only those 9 lines of Python code:

artist = Artist.fetchone(pgconn, name=sys.argv[ 1 ]) for album in Album.fetchall(pgconn, artistid=artist. id ): ms = 0 for track in Track.fetchall(pgconn, albumid=album. id ): ms += track.duration duration = timedelta(milliseconds=ms) print ( " %25s : %s " % (album.title, duration))

With the equivalent query written as:

select album.title as album, sum (milliseconds) * interval '1 ms' as duration from album join artist using (artistid) left join track using (albumid) where artist.name = 'Red Hot Chili Peppers' group by album order by album;

It seems to me that the expected maintenance burden of both the solution are roughly equivalent. The main difference is that in the second case PostgreSQL is smart enough to pick the proper join algorithm depending on the size of the different data sets involved and the defined indexes, where in the Python code we manually express a nested loop and would have to manually code up a hash join or a merge join should the data set size come to require it.

A somewhat interesting maintenance case to consider would be to change the result ordering. Say now we want to display the album list sorted by album’s duration, shortest first.

Exercice to the reader: implement the new ordering specification in the application’s code, where you deal with business logic. Note that the Chinook database model does not have a per-album duration so you can’t rely on .orderBy(duration) or something like that at your ORM layer.

In the SQL solution, this translates to changing the order by clause of course, and the change is easy for your peers to review before your commit makes it to the release branch:

-order by album; +order by duration;

Unfortunately we don’t have the album’s date of release in the Chinook database model, that would be the proper ordering here I guess.

Conclusion

Of course SQL is meant to implement business logic, and that doesn’t mean you need to resort to Stored Procedure to do so. I find that in too many cases, modern developers tend to forget about basic application’s architecture and like to pretend they can live with a single application’s programming language in the backend.

So it’s time to properly learn SQL and use it to its full potential as part of your backend source code, as we saw in How to Write SQL previously here. And now that you consider SQL as code, also about how to do SQL Regression Tests. And remember, if you think education is expensive, try ignorance.