Add NULLS LAST to index on merge request metrics

To improve the sort performance

changelogs/unreleased/add-nulls-last-to-db-index.yml

+---
+title: Add NULLS LAST to index on merge request metrics
+merge_request: 47300
+author:
+type: performance

db/migrate/20201110133629_change_index_mr_metrics_target_project_id.rb

+# frozen_string_literal: true
+
+class ChangeIndexMrMetricsTargetProjectId < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_merge_request_metrics_on_target_project_id_merged_at'
+  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
+    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
+    remove_concurrent_index_by_name(:merge_request_metrics, NULLS_LAST_INDEX_NAME)
+  end
+end

db/schema_migrations/20201110133629

+83773b825db9b2671fd4ffb2c0d6733036737385ce7a933040011026b34ba1e1
\ No newline at end of file

db/structure.sql

@@ -21108,8 +21108,6 @@ CREATE INDEX index_merge_request_metrics_on_pipeline_id ON merge_request_metrics
 
 CREATE INDEX index_merge_request_metrics_on_target_project_id ON merge_request_metrics USING btree (target_project_id);
 
-CREATE INDEX index_merge_request_metrics_on_target_project_id_merged_at ON merge_request_metrics USING btree (target_project_id, merged_at);
-
 CREATE UNIQUE INDEX index_merge_request_reviewers_on_merge_request_id_and_user_id ON merge_request_reviewers USING btree (merge_request_id, user_id);
 
 CREATE INDEX index_merge_request_reviewers_on_user_id ON merge_request_reviewers USING btree (user_id);
@@ -21194,6 +21192,8 @@ 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 UNIQUE INDEX index_namespace_aggregation_schedules_on_namespace_id ON namespace_aggregation_schedules USING btree (namespace_id);
 
 CREATE UNIQUE INDEX index_namespace_root_storage_statistics_on_namespace_id ON namespace_root_storage_statistics USING btree (namespace_id);