Add index for relative positions on designs

In our main finder queries we're filtering by `issue_id`, and then
sorting by `relative_position` and `id` in ascending order.

By adding a combined index on `(issue_id, relative_position, id)` we
can use this index for both filtering and sorting.

db/migrate/20200728174137_add_index_on_design_management_designs_issue_id_and_relative_position_and_id.rb

+# frozen_string_literal: true
+
+class AddIndexOnDesignManagementDesignsIssueIdAndRelativePositionAndId < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_design_management_designs_issue_id_relative_position_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :design_management_designs, [:issue_id, :relative_position, :id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :design_management_designs, [:issue_id, :relative_position, :id], name: INDEX_NAME
+  end
+end

db/schema_migrations/20200728174137

+a16e7fdcc62f39af3038317cb39ffb4c35f41ae45f5de429f18837309739110b
\ No newline at end of file

db/structure.sql

@@ -19385,6 +19385,8 @@ CREATE INDEX index_description_versions_on_issue_id ON public.description_versio
 
 CREATE INDEX index_description_versions_on_merge_request_id ON public.description_versions USING btree (merge_request_id) WHERE (merge_request_id IS NOT NULL);
 
+CREATE INDEX index_design_management_designs_issue_id_relative_position_id ON public.design_management_designs USING btree (issue_id, relative_position, id);
+
 CREATE UNIQUE INDEX index_design_management_designs_on_issue_id_and_filename ON public.design_management_designs USING btree (issue_id, filename);
 
 CREATE INDEX index_design_management_designs_on_project_id ON public.design_management_designs USING btree (project_id);