Introduce compound index on vulnerabilities to speed up migration

db/migrate/20200826220745_add_compound_index_on_vulnerabilities_for_background_migration.rb

+# frozen_string_literal: true
+
+class AddCompoundIndexOnVulnerabilitiesForBackgroundMigration < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_vulnerabilities_on_project_id_and_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :vulnerabilities, [:project_id, :id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :vulnerabilities, INDEX_NAME
+  end
+end

db/schema_migrations/20200826220745

+ee38dd60087a8879c4686214da1d25a60ab74306eb07b938efb1a8dfc46cc73a
\ No newline at end of file

db/structure.sql

@@ -21038,6 +21038,8 @@ CREATE INDEX index_vulnerabilities_on_milestone_id ON public.vulnerabilities USI
 
 CREATE INDEX index_vulnerabilities_on_project_id ON public.vulnerabilities USING btree (project_id);
 
+CREATE INDEX index_vulnerabilities_on_project_id_and_id ON public.vulnerabilities USING btree (project_id, id);
+
 CREATE INDEX index_vulnerabilities_on_resolved_by_id ON public.vulnerabilities USING btree (resolved_by_id);
 
 CREATE INDEX index_vulnerabilities_on_start_date_sourcing_milestone_id ON public.vulnerabilities USING btree (start_date_sourcing_milestone_id);