The index is supposed to be created automatically when you run rails db:migrate. What's the output of rails db:migrate:status (its output is quite lengthy, so it'll be easier to run rails db:migrate:status | grep "fulltext")? If the index is supposed to be there, you'll see something along the lines of up 20191207001359 Add search fulltext index to posts. If the migration is present in the output and its status is "up", it's probably a bug of ours.

Thanks for your help. I have run rails db:migrate:status|grep "fulltext" and I get:

   up     20191207001359  Add search fulltext index to posts

(and all other migrations have the status "up" too)

Is there any immediate solution, like running the migration step again manually or running some SQL statements directly in the database?

The output looks fine, yeah. Let's see if anything else is amiss (as soon as we establish what exactly is wrong, I'll be able to advise on what to do to resolve it) - select index_schema, table_name, index_name, column_name, collation, is_visible from information_schema.statistics where index_type = 'FULLTEXT'; (I included index_schema for clarity - it'll contain your database names, so you'll likely want to edit them out) should show whether the necessary indexes exist and are correctly configured. A good output should contain at least two entries for the index_posts_on_body_markdown_and_title index with column names body_markdown and title (is_visible should be YES on both of the columns too).