Today I Learned

Partial indexes: index the column you sort by

Sat, 12 Sep 2026 00:00:00 GMT

A background worker regularly fetched the oldest flagged rows from a table with millions of records:

SELECT * FROM posts WHERE needs_sync = true ORDER BY updated_at LIMIT 200;

Only a small fraction of the records were flagged, so I started with a partial index on the flag:

CREATE INDEX ON posts (needs_sync) WHERE needs_sync = true;

This turned out to be less useful than I expected. Inside such an index every key is true, because the value is taken from needs_sync. The index can be used to find the rows, but cannot be used for sorting. So to get the oldest 200 rows, Postgres still has to fetch all flagged rows and sort them before it can apply the LIMIT.

Put the column you ORDER BY in the key and the flag in the WHERE:

CREATE INDEX ON posts (updated_at) WHERE needs_sync = true;

The index scan can now stop after 200 entries. It can also be used for a fast index-only scan when counting the flagged rows:

SELECT COUNT(*) FROM posts WHERE needs_sync = true;

Shim a missing libc header on Alpine

Mon, 09 Oct 2023 00:00:00 GMT

In an elixir:*-alpine image, mix deps.compile failed on a native dependency (picosat_elixir, the SAT-solver NIF that Ash uses for policies):

c_src/picosat.c:8150:10: fatal error: sys/unistd.h: No such file or directory

<sys/unistd.h> is a glibc quirk. All it does is include <unistd.h>. The latter is the POSIX header, so it’s the only one that musl provides.

There’s no need to fork picosat_elixir to fix this. I just added the same forwarding header to the image:

Dockerfile

RUN echo "#include <unistd.h>" > /usr/include/sys/unistd.h

The trick works for other missing glibc forwarding headers too, as long as musl has the header they forward to.

Other options:

Use Debian. Phoenix’s own generated Dockerfile already does.
For Ash specifically, replace {:picosat_elixir, "~> 0.2"} with the pure-Elixir {:simple_sat, "~> 0.1"}. It’s a bit slower but avoids NIF-compilation issues entirely.

Simple static web server

Wed, 15 Mar 2023 00:00:00 GMT

Sometimes you need to quickly serve some static files for a preview. The good news is that you don’t have to install or configure a web server for that. If you have Python installed on your machine (which you probably do), there’s already a handy little web server ready for you.

Just run this command to serve the current folder:

python3 -m http.server 8080

The caveat is that it doesn’t support the live reload. So if you need that, a better option could be live-server from npm.

npx live-server --port=8080