The Postgres REPL

R0ml Lefkowitz’s The Image of Postgres evokes the Smalltalk experience: reach deeply into a running system, make small changes, see immediate results. There isn’t yet a fullblown IDE for the style of Postgres-based development I describe in this series, though I can envision a VSCode extension that would provide one. But there is certainly a REPL (read-eval-print loop), it’s called psql, and it delivers the kind of immediacy that all REPLs do. In our case there’s also Metabase; it offers a complementary REPL that enhances its power as a lightweight app server.

In the Clojure docs it says:

The Clojure REPL gives the programmer an interactive development experience. When developing new functionality, it enables her to build programs first by performing small tasks manually, as if she were the computer, then gradually make them more and more automated, until the desired functionality is fully programmed. When debugging, the REPL makes the execution of her programs feel tangible: it enables the programmer to rapidly reproduce the problem, observe its symptoms closely, then improvise experiments to rapidly narrow down the cause of the bug and iterate towards a fix.

I feel the same way about the Python REPL, the browser’s REPL, the Metabase REPL, and now also the Postgres REPL. Every function and every materialized view in the analytics system begins as a snippet of code pasted into the psql console (or Metabase). Iteration yields successive results instantly, and those results reflect live data. In How is a Programmer Like a Pathologist Gilad Bracha wrote:

A live program is dynamic; it changes over time; it is animated. A program is alive when it’s running. When you work on a program in a text editor, it is dead.

Tudor Girba amplified the point in a tweet.

In a database-backed system there’s no more direct way to interact with live data than to do so in the database. The Postgres REPL is, of course, a very sharp tool. Here are some ways to handle it carefully.

Find the right balance for tracking incremental change

In Working in a hybrid Metabase / Postgres code base I described how version-controlled files — for Postgres functions and views, and for Metabase questions — repose in GitHub and drive a concordance of docs. I sometimes write code snippets directly in psql or Metabase, but mainly compose in a “repository” (telling word!) where those snippets are “dead” artifacts in a text editor. They come to life when pasted into psql.

A knock on Smalltalk was that it didn’t play nicely with version control. If you focus on the REPL aspect, you could say the same of Python or JavaScript. In any such case there’s a balance to be struck between iterating at the speed of thought and tracking incremental change. Working solo I’ve been inclined toward a fairly granular commit history. In a team context I’d want to leave a chunkier history but still record the ongoing narrative somewhere.

Make it easy to understand the scope and effects of changes

The doc concordance has been the main way I visualize interdependent Postgres functions, Postgres views, and Metabase questions. In Working with interdependent Postgres functions and materialized views I mentioned Laurenz Albe’s Tracking View Dependencies in Postgres. I’ve adapted the view dependency tracker he develops there, and adapted related work from others to track function dependencies.

This tooling is still a work in progress, though. The concordance doesn’t yet include Postgres types, for example, nor the tables that are upstream from materialized views. My hypothetical VSCode extension would know about all the artifacts and react immediately when things change.

Make it easy to find and discard unwanted artifacts

Given a function or view named foo, I’ll often write and test a foo2 before transplanting changes back into foo. Because foo may often depend on bar and call baz I wind up also with bar2 and baz2. These artifacts hang around in Postgres until you delete them, which I try to do as I go along.

If foo2 is a memoized function (see this episode), it can be necessary to delete the set of views that it’s going to recreate. I find these with a query.

  'drop materialized view ' || matviewname || ';' as drop_stmt
from pg_matviews 
where matviewname ~* {{ pattern }}

That pattern might be question_and_answer_summary_for_group to find all views based on that function, or _6djxg2yk to find all views for a group, or even [^_]{8,8}$ to find all views made by memoized functions.

I haven’t yet automated the discovery or removal of stale artifacts and references to them. That’s another nice-to-have for the hypothetical IDE.

The Image of Postgres

I’ll give R0ml the last word on this topic.

This is the BYTE magazine cover from August of 1981. In the 70s and the 80s, programming languages had this sort of unique perspective that’s completely lost to history. The way it worked: a programming environment was a virtual machine image, it was a complete copy of your entire virtual machine memory and that was called the image. And then you loaded that up and it had all your functions and your data in it, and then you ran that for a while until you were sort of done and then you saved it out. And this wasn’t just Smalltalk, Lisp worked that way, APL worked that way, it was kind of like Docker only it wasn’t a separate thing because everything worked that way and so you didn’t worry very much about persistence because it was implied. If you had a programming environment it saved everything that you were doing in the programming environment, you didn’t have to separate that part out. A programming environment was a place where you kept all your data and business logic forever.

So then Postgres is kind of like Smalltalk only different.

What’s the difference? Well we took the UI out of Smalltalk and put it in the browser. The rest of it is the same, so really Postgres is an application delivery platform, just like we had back in the 80s.


9 thoughts on “The Postgres REPL

  1. I remember getting Forth working on my first PC. I had worked with Forth for several years on a CP/M machine. Forth had all those traits you mention. The basic Forth read from and wrote to raw disk sectors, and had only enough of an editor to edit a line at a time. But code was out there for a “full screen editor”, which I adapted and got working. Then little by little I modified it in place, as you could do in Forth, to call BIOS interrupts which let me make DOS calls. With that I could use DOS to talk to the disk system, and I converted the disk storage to use regular DOS files instead of sectors.

    It was done step-by-step on a running system, and there was no practical way to go back except to start again from scratch. Fun, fun!

    1. A real challenge here is to differentiate between nostalgia for things of the past and appreciation of things that are (or should be) timeless.

      1. Nostalgia / timeless, Exactly! Which category to put references to Small Talk / Lisp / Clojure in? Do current CS graduates know anything about these historic references?

        I’m pretty sure references to Byte magazine fall into the nostalgia bin : )

        And having being coding / hacking since 1977 I’m not sure what to think when I had to look up “memoized”. Have I led a sheltered life and need to get out more?

  2. Clojure is too young to be historic, and that’s why I included that reference: for hipster appeal!

    Thanks for asking, I added a link to Wikipedia’s definition of memoization, and a link to the episode in this series where I describe a Postgres implementation.

  3. So many things resonate with me in this post. I’m also given to suddenly suggesting that in a related way, the shell is also a REPL. And if one considers the file system as being the data and persistence, it’s similar to how R0ml describes it. Moreover, if the Python or Closure REPLs (both of which I’ve enjoyed) have low friction between the immediate mode and the language, then something like Bash has zero friction.

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