Add A Database Index If It Does Not Already Exist

Sometimes you aren't sure if an index might already exist in one of the environments where a migration is going to run. But you still need to add the index elsewhere. One way of handling that is to add an index with the if_not_exists directive.

class AddIndexToEventsCreatedAt < ActiveRecord::Migration[6.1]
  def change
    add_index :events, :created_at, if_not_exists: true
  end
end

ActiveRecord will translate this directive into the resulting SQL statement like so:

create index if not exists index_events_on_created_at on events ... ;

This way the index will be created in a database where it doesn't already exist and otherwise the statement will short-circuit rather than erroring when one does exist.

A couple notes:

1. From the PostgreSQL manual:

there is no guarantee that the existing index is anything like the one that would have been created.

2. The if_not_exists option also works with create_table.

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