CREATE INDEX on a table without CONCURRENTLY — locks writes during deploy

What it is

A migration creates an index without the CONCURRENTLY keyword.

Why it matters

A plain CREATE INDEX takes a write lock for the whole build. On a large table that is minutes of failed writes during deploy — an outage caused by a change intended to improve performance.

How to fix it

Use CREATE INDEX CONCURRENTLY. It cannot run inside a transaction block, so put it in its own migration, and check for an INVALID index afterwards if the build fails partway.