An Architect's View

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SQL NULL, S/nilable, and Optionality

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Rich Hickey gave a very thought-provoking talk at Clojure/conj 2018 called Maybe Not, where he mused on optionality and how we represent the absence of a value.

His talk covered many things, including how clojure.spec/keys currently complects both structure and optionality (and his thoughts on fixing that in a future version of clojure.spec), but his mention of s/nilable was what triggered an “ah-ha!” moment for me.

At World Singles Networks, we deal with a lot of data in SQL (specifically in Percona’s fork of MySQL) and, in SQL, you represent the absence of a value with NULL in a column. Columns that represent optional data must be declared as nullable and when you read data from them with clojure.java.jdbc you get hash map entries in the rows that have nil values. If you’re using clojure.spec to describe your tables, rows, and columns, then you are going to have lots of s/nilable specs – and now your “optionality” has been reified into nil values, cast in the stone of your specs… which is clearly not an ideal situation!

This made me realize that java.jdbc probably should just omit keys whose values represent SQL NULL. They are, after all, optional values rather than truly nilable values.

That would be a potentially breaking change in behavior for java.jdbc users. Sure, in most cases, if you have a hash map representing a row in a database table, you’re not really going to care whether (:col row) gives you nil because :col maps to nil or because row doesn’t contain :col. There are use cases where it matters: contains?, row/column specs, tabular printing.

Along with changing the behavior of NULL columns and supporting datafy and nav, I have a lot of other changes that I’d like to apply to java.jdbc, such as automatically qualifying column keys with the table from which they came, improving overall performance (by no longer converting ResultSet objects to sequences of hash maps), dramatically simplifying and streamlining the options that are available (since many of them are very rarely used), and focusing on a reducible-first API. All of which would be breaking changes.

I’ve learned a lot – about Clojure, idioms, and databases – over the seven years that I’ve been maintaining org.clojure/java.jdbc, and it is time for a new namespace or perhaps even a completely new project, that offers a better way to deal with SQL databases from Clojure! I’ll be writing a series of blog posts about the differences I envisage between the current de facto standard JDBC wrapper and where I’d like to go with this, so that I can get community feedback on what should stay, what should change, and what should go. Stay tuned!

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