Add partial index on locked merge requests

In https://gitlab.com/gitlab-org/gitlab/-/issues/220771, the
`/api/v4/internal/allowed` endpoint would timeout after 60 seconds due
to this query in `MatchingMergeRequest#match?`.

This commit adds a partial index for locked merge requests to improve
the performance of this query.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/220771

changelogs/unreleased/sh-add-partial-index-locked-merge-requests.yml

+---
+title: Add partial index on locked merge requets
+merge_request: 34127
+author:
+type: performance

db/post_migrate/20200609002841_add_partial_index_on_locked_state_id_to_merge_requests.rb

+# frozen_string_literal: true
+
+class AddPartialIndexOnLockedStateIdToMergeRequests < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = "idx_merge_requests_on_target_project_id_and_locked_state"
+  LOCKED_STATE_ID = 4
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, :target_project_id, where: "(state_id = #{LOCKED_STATE_ID})", name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :merge_requests, name: INDEX_NAME
+  end
+end

db/structure.sql

@@ -9119,6 +9119,8 @@ CREATE INDEX idx_merge_requests_on_state_id_and_merge_status ON public.merge_req
 
 CREATE INDEX idx_merge_requests_on_target_project_id_and_iid_opened ON public.merge_requests USING btree (target_project_id, iid) WHERE (state_id = 1);
 
+CREATE INDEX idx_merge_requests_on_target_project_id_and_locked_state ON public.merge_requests USING btree (target_project_id) WHERE (state_id = 4);
+
 CREATE UNIQUE INDEX idx_metrics_users_starred_dashboard_on_user_project_dashboard ON public.metrics_users_starred_dashboards USING btree (user_id, project_id, dashboard_path);
 
 CREATE INDEX idx_mr_cc_diff_files_on_mr_cc_id_and_sha ON public.merge_request_context_commit_diff_files USING btree (merge_request_context_commit_id, sha);
@@ -13847,5 +13849,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200603073101
 20200604143628
 20200608075553
+20200609002841
 \.