#data-structures

Programming Languages Bug Report

An import job timed out at 100k rows because of one `in` check on a list

Julian Navarro · Nov 9, 2026 · 7 min read

A customer import that took seconds in testing ran for over an hour on a real file and hit the worker timeout. The cause was a membership test on a list inside a loop. How we found it with a profiler, measured the fix with timeit, and guarded against it coming back.

Programming Languages

The data structures behind caches, queues and indexes in system design

John Nguyen · Apr 4, 2026 · 9 min read

An LRU cache is a hash map plus a linked list. A database index is a B-tree. A rate limiter is often a sorted set. When we started reading system components as data structures, capacity questions and odd latency curves became much easier to explain.