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How to Create Postgres Indexes Concurrently in ActiveRecord Migrations

By default, Postgres locks writes (but not reads) to a table while creating an index on it. That can result in unacceptable downtime during a production deploy. On a large table, indexing can take hours.

However, Postgres has a CONCURRENTLY option for CREATE INDEX that creates the index without preventing concurrent INSERTs, UPDATEs, or DELETEs on the table.

To make this option easier to use in migrations, ActiveRecord 4 introduced an algorithm: :concurrently option for add_index.

Here’s an example:

class AddIndexToAsksActive < ActiveRecord::Migration
  disable_ddl_transaction!

  def change
    add_index :asks, :active, algorithm: :concurrently
  end
end

The caveat is that concurrent indexes must be created outside a transaction. By default, ActiveRecord migrations are run inside a transaction.

ActiveRecord 4’s new disable_ddl_transaction! method must therefore be used in combination with algorithm: :concurrently migrations (as seen above).

The disable_ddl_transaction! method applies only to that migration file. Adjacent migrations still run in their own transactions and roll back automatically if they fail. Therefore, it’s a good idea to isolate concurrent index migrations to their own migration files.

Use this technique with ActiveRecord 4 to create Postgres indexes concurrently and avoid accidental downtime caused by long, write-blocking database indexing.