Selecting duplicate records might be useful in cases when you want to introduce newmulti column index on existing data.

duplicates=ActiveRecord::Base.connection.execute('  SELECT items.id, items.name    FROM items    WHERE items.id IN (      SELECT        id        FROM (          SELECT          id,          ROW_NUMBER() OVER w AS rnum          FROM items          WINDOW w AS (            PARTITION BY entity_id, barcode            ORDER BY id            )          ) t          WHERE t.rnum > 1       );    ')

Our exported data set will haveid andname, so that we can tell which rows we want to delete. And it will only show rows whereentity_id andbarcode wouldnot be unique. That's an example, yours will be different.

After the cleanup, when the query above would return an empty result set we should be able to create following index without any issues:

add_index(:items,[:entity_id,:barcode])

Inspiration taken from the articleRails: migration: Adding a unique Index and deleting Duplicates

Image generated via Midjourney prompt:Heap of items where every object is represented twice; --ar 16:9