Glossary: System Design Beyond the Basics

Idempotent insert (primary key, INSERT OR IGNORE)

Also written: INSERT OR IGNORE, Primary key, make_shard()

A primary key is a column whose value may appear only once in a table, so each order ID exists once. sqlite's `INSERT OR IGNORE` silently skips a row whose key already exists, so a retried order is stored once; the prototype's `make_shard()` builds an in-memory database with such an `orders` table.