Fix index definition

Add `id` to the index to support the tie breaker ordering.

db/migrate/20201110133629_change_index_mr_metrics_target_project_id.rb

@@ -9,12 +9,12 @@ class ChangeIndexMrMetricsTargetProjectId < ActiveRecord::Migration[6.0]
   NULLS_LAST_INDEX_NAME = 'index_mr_metrics_on_target_project_id_merged_at_nulls_last'
 
   def up
-    add_concurrent_index :merge_request_metrics, [:target_project_id, :merged_at], order: { merged_at: 'DESC NULLS LAST' }, name: NULLS_LAST_INDEX_NAME
+    add_concurrent_index :merge_request_metrics, [:target_project_id, :merged_at, :id], order: { merged_at: 'DESC NULLS LAST', id: 'DESC' }, name: NULLS_LAST_INDEX_NAME
     remove_concurrent_index_by_name(:merge_request_metrics, INDEX_NAME)
   end
 
   def down
-    add_concurrent_index :merge_request_metrics, [:target_project_id, :created_at, :merged_at], name: INDEX_NAME
+    add_concurrent_index :merge_request_metrics, [:target_project_id, :merged_at], name: INDEX_NAME
     remove_concurrent_index_by_name(:merge_request_metrics, NULLS_LAST_INDEX_NAME)
   end
 end

db/structure.sql

@@ -21192,7 +21192,7 @@ CREATE UNIQUE INDEX index_mr_blocks_on_blocking_and_blocked_mr_ids ON merge_requ
 
 CREATE UNIQUE INDEX index_mr_context_commits_on_merge_request_id_and_sha ON merge_request_context_commits USING btree (merge_request_id, sha);
 
-CREATE INDEX index_mr_metrics_on_target_project_id_merged_at_nulls_last ON merge_request_metrics USING btree (target_project_id, merged_at DESC NULLS LAST);
+CREATE INDEX index_mr_metrics_on_target_project_id_merged_at_nulls_last ON merge_request_metrics USING btree (target_project_id, merged_at DESC NULLS LAST, id DESC);
 
 CREATE UNIQUE INDEX index_namespace_aggregation_schedules_on_namespace_id ON namespace_aggregation_schedules USING btree (namespace_id);