```ruby
# See http://doc.gitlab.com/ce/development/migration_style_guide.html
# for more information on how to write migrations for GitLab.

class RemoveDuplicatesFromRoutes < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  def up
    # We can skip this migration when running a PostgreSQL database because
    # we use an optimized query in the "FillProjectsRoutesTable" migration
    # to fill these values that avoid duplicate entries in the routes table.
    return unless Gitlab::Database.mysql?

    select_all("SELECT path FROM #{quote_table_name(:routes)} GROUP BY path HAVING COUNT(*) > 1").each do |row|
      path = connection.quote(row['path'])
      execute(%Q{
        DELETE FROM #{quote_table_name(:routes)}
        WHERE path = #{path}
        AND id != (
          SELECT id FROM (
            SELECT max(id) AS id
            FROM #{quote_table_name(:routes)}
            WHERE path = #{path}
          ) max_ids
        )
      })
    end
  end

  def down
  end
end
```

Improve performance on RemoveDuplicatesFromRoutes migration
user
```ruby
# See http://doc.gitlab.com/ce/development/migration_style_guide.html
# for more information on how to write migrations for GitLab.

class RemoveDuplicatesFromRoutes < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  def up
    # We can skip this migration when running a PostgreSQL database because
    # we use an optimized query in the "FillProjectsRoutesTable" migration
    # to fill these values that avoid duplicate entries in the routes table.
    return unless Gitlab::Database.mysql?

    execute <<-EOF
      DELETE duplicated_rows.*
      FROM routes AS duplicated_rows
        INNER JOIN (
          SELECT path, MAX(id) as max_id
          FROM routes
          GROUP BY path
          HAVING COUNT(*) > 1
        ) AS good_rows ON good_rows.path = duplicated_rows.path AND good_rows.max_id <> duplicated_rows.id;
    EOF
  end

  def down
  end
end
```
assistant