Change PrometheusMetrics Identifier index

Enforce uniqueness for PrometheusMetrics identifier when project_id is
null.

changelogs/unreleased/rc-remove_metric_identifier_index.yml

+---
+title: Change PrometheusMetrics identifier index
+merge_request: 35912
+author:
+type: fixed

db/migrate/20200702201039_change_prometheus_metrics_identifier_index.rb

+# frozen_string_literal: true
+
+class ChangePrometheusMetricsIdentifierIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  NEW_INDEX = :index_prometheus_metrics_on_identifier_and_null_project
+  OLD_INDEX = :index_prometheus_metrics_on_identifier
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index            :prometheus_metrics, :identifier, name: NEW_INDEX, unique: true, where: 'project_id IS NULL'
+    remove_concurrent_index_by_name :prometheus_metrics, OLD_INDEX
+  end
+
+  def down
+    add_concurrent_index            :prometheus_metrics, :identifier, name: OLD_INDEX, unique: true
+    remove_concurrent_index_by_name :prometheus_metrics, NEW_INDEX
+  end
+end

db/structure.sql

@@ -20025,7 +20025,7 @@ CREATE INDEX index_prometheus_metrics_on_common ON public.prometheus_metrics USI
 
 CREATE INDEX index_prometheus_metrics_on_group ON public.prometheus_metrics USING btree ("group");
 
-CREATE UNIQUE INDEX index_prometheus_metrics_on_identifier ON public.prometheus_metrics USING btree (identifier);
+CREATE UNIQUE INDEX index_prometheus_metrics_on_identifier_and_null_project ON public.prometheus_metrics USING btree (identifier) WHERE (project_id IS NULL);
 
 CREATE UNIQUE INDEX index_prometheus_metrics_on_identifier_and_project_id ON public.prometheus_metrics USING btree (identifier, project_id);
 
@@ -23718,6 +23718,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200701093859
 20200701205710
 20200702123805
+20200702201039
 20200703064117
 20200703121557
 20200703154822