Postgres is out of headroom. How do you choose a shard key, and how do you reshard later without downtime?

Sharding buys write headroom and bills you in joins, foreign keys, and every future migration. The key choice is a query-pattern bet; the resharding choreography separates operators from diagram artists.

Updated Sep 2026 · Grounded in researched DevOps, SRE and platform engineering interview loops, written to a senior-engineer editorial bar, and never padded to hit a word count.