Replace index for service usage data

There is an index index_services_on_type_and_id_and_template_when_active
introduced in 3a44d110 to optimize the queries performance for the
services usage data.

The usage data counter is iterating over the index in 10K batches,
without covering all the fields (type, id, template, instance), the
database would need to constantly look at the table for the instance
column (extra I/O).

changelogs/unreleased/204802-exclude-instance-services-from-usage-data.yml

+---
+title: Replace index for service usage data
+merge_request: 38147
+author:
+type: performance

db/migrate/20200730083043_replace_index_for_service_usage_data.rb

+# frozen_string_literal: true
+
+class ReplaceIndexForServiceUsageData < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  OLD_INDEX_NAME = 'index_services_on_type_and_id_and_template_when_active'
+  NEW_INDEX_NAME = 'index_services_on_type_id_when_active_not_instance_not_template'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :services, [:type, :id], where: 'active = TRUE AND instance = FALSE AND template = FALSE', name: NEW_INDEX_NAME
+
+    remove_concurrent_index_by_name :services, OLD_INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :services, [:type, :id, :template], where: 'active = TRUE', name: OLD_INDEX_NAME
+
+    remove_concurrent_index :services, NEW_INDEX_NAME
+  end
+end

db/schema_migrations/20200730083043

+5bda9ae726077c1ea0df2158138f85ca43bfbf5b1a8cf3516bcdf012eac5c0b8
\ No newline at end of file

db/structure.sql

@@ -20523,12 +20523,12 @@ CREATE INDEX index_services_on_template ON public.services USING btree (template
 
 CREATE INDEX index_services_on_type ON public.services USING btree (type);
 
-CREATE INDEX index_services_on_type_and_id_and_template_when_active ON public.services USING btree (type, id, template) WHERE (active = true);
-
 CREATE UNIQUE INDEX index_services_on_type_and_instance_partial ON public.services USING btree (type, instance) WHERE (instance = true);
 
 CREATE UNIQUE INDEX index_services_on_type_and_template_partial ON public.services USING btree (type, template) WHERE (template = true);
 
+CREATE INDEX index_services_on_type_id_when_active_not_instance_not_template ON public.services USING btree (type, id) WHERE ((active = true) AND (instance = false) AND (template = false));
+
 CREATE UNIQUE INDEX index_shards_on_name ON public.shards USING btree (name);
 
 CREATE INDEX index_slack_integrations_on_service_id ON public.slack_integrations USING btree (service_id);