How to Create Postgres Indexes Concurrently in ActiveRecord Migrations
https://thoughtbot.com/blog/how-to-create-postgres-indexes-concurrently-inPostgreSQL at Scale: Database Schema Changes Without Downtime
https://medium.com/braintree-product-technology/postgresql-at-scale-database-schema-changes-without-downtime-20d3749ed680PostgreSQL rocks, except when it blocks: Understanding locks
https://www.citusdata.com/blog/2018/02/15/when-postgresql-blocks/Postgres: Adding Foreign Keys With Zero Downtime — Kitty Con Gato
https://travisofthenorth.com/blog/2017/2/2/postgres-adding-foreign-keys-with-zero-downtimeZero-downtime schema migrations in Postgres using views – Fabian Lindfors
https://fabianlindfors.se/blog/schema-migrations-in-postgres/