Remove non-unique index on MR metrics

changelogs/unreleased/drop-old-non-unique-index-on-mr-metrics.yml

+---
+title: Remove non-unique index on `merge_request_metrics.merge_request_id` column
+merge_request: 36170
+author:
+type: changed

db/migrate/20200707071941_drop_old_non_unique_index_on_mr_metrics.rb

+# frozen_string_literal: true
+
+class DropOldNonUniqueIndexOnMrMetrics < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_merge_request_metrics'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name(:merge_request_metrics, INDEX_NAME)
+  end
+
+  def down
+    add_concurrent_index :merge_request_metrics, :merge_request_id, name: INDEX_NAME
+  end
+end

db/structure.sql

@@ -19437,8 +19437,6 @@ CREATE INDEX index_merge_request_diffs_on_merge_request_id_and_id ON public.merg
 
 CREATE INDEX index_merge_request_diffs_on_merge_request_id_and_id_partial ON public.merge_request_diffs USING btree (merge_request_id, id) WHERE ((NOT stored_externally) OR (stored_externally IS NULL));
 
-CREATE INDEX index_merge_request_metrics ON public.merge_request_metrics USING btree (merge_request_id);
-
 CREATE INDEX index_merge_request_metrics_on_first_deployed_to_production_at ON public.merge_request_metrics USING btree (first_deployed_to_production_at);
 
 CREATE INDEX index_merge_request_metrics_on_latest_closed_at ON public.merge_request_metrics USING btree (latest_closed_at) WHERE (latest_closed_at IS NOT NULL);
@@ -23582,5 +23580,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200704143633
 20200706005325
 20200706170536
+20200707071941
 \.