Add verification indexes for package files

For queries produced by these methods specifically:

- Packages::PackageFile.verification_pending_batch
- Packages::PackageFile.verification_failed_batch
- Packages::PackageFile.needs_verification_count

Since they are always running, at least every minute, and potentially
with high concurrency (depending on admin settings).

changelogs/unreleased/mk-geo-primary-backfills-checksums.yml

+---
+title: 'Geo: Add verification indexes for package files'
+merge_request: 47372
+author:
+type: added

db/migrate/20201201033202_add_verification_indexes_for_package_files.rb

+# frozen_string_literal: true
+
+class AddVerificationIndexesForPackageFiles < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  PENDING_VERIFICATION_INDEX_NAME = "packages_packages_pending_verification"
+  FAILED_VERIFICATION_INDEX_NAME = "packages_packages_failed_verification"
+  NEEDS_VERIFICATION_INDEX_NAME = "packages_packages_needs_verification"
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :packages_package_files, :verified_at, where: "(verification_state = 0)", order: { verified_at: 'ASC NULLS FIRST' }, name: PENDING_VERIFICATION_INDEX_NAME
+    add_concurrent_index :packages_package_files, :verification_retry_at, where: "(verification_state = 3)", order: { verification_retry_at: 'ASC NULLS FIRST' }, name: FAILED_VERIFICATION_INDEX_NAME
+    add_concurrent_index :packages_package_files, :verification_state, where: "(verification_state = 0 OR verification_state = 3)", name: NEEDS_VERIFICATION_INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :packages_package_files, PENDING_VERIFICATION_INDEX_NAME
+    remove_concurrent_index_by_name :packages_package_files, FAILED_VERIFICATION_INDEX_NAME
+    remove_concurrent_index_by_name :packages_package_files, NEEDS_VERIFICATION_INDEX_NAME
+  end
+end

db/schema_migrations/20201201033202

+418481f8281f8908740d3a8378b420f4d83853aab139b3401f7e410fc97d2488
\ No newline at end of file

db/structure.sql

@@ -22741,6 +22741,12 @@ CREATE UNIQUE INDEX one_canonical_wiki_page_slug_per_metadata ON wiki_page_slugs
 
 CREATE INDEX package_name_index ON packages_packages USING btree (name);
 
+CREATE INDEX packages_packages_failed_verification ON packages_package_files USING btree (verification_retry_at NULLS FIRST) WHERE (verification_state = 3);
+
+CREATE INDEX packages_packages_needs_verification ON packages_package_files USING btree (verification_state) WHERE ((verification_state = 0) OR (verification_state = 3));
+
+CREATE INDEX packages_packages_pending_verification ON packages_package_files USING btree (verified_at NULLS FIRST) WHERE (verification_state = 0);
+
 CREATE INDEX partial_index_ci_builds_on_scheduled_at_with_scheduled_jobs ON ci_builds USING btree (scheduled_at) WHERE ((scheduled_at IS NOT NULL) AND ((type)::text = 'Ci::Build'::text) AND ((status)::text = 'scheduled'::text));
 
 CREATE INDEX partial_index_deployments_for_legacy_successful_deployments ON deployments USING btree (id) WHERE ((finished_at IS NULL) AND (status = 2));