Skip projects filter on merge requests search

When searching for merge requests, an additional subquery
is added which by default filters only merge requests which belong
to source or target project user has permission for.

This filter is not needed because more restrictive filter
which checks if user has permission for target project
is used in the query.

So unless a custom projects filter is used by user, it's possible
to skip the default projects filter and speed up the final query.

Related to #40540

app/services/search/global_service.rb

 module Search
   class GlobalService
     attr_accessor :current_user, :params
+    attr_reader :default_project_filter
 
     def initialize(user, params)
       @current_user, @params = user, params.dup
+      @default_project_filter = true
     end
 
     def execute
-      Gitlab::SearchResults.new(current_user, projects, params[:search])
+      Gitlab::SearchResults.new(current_user, projects, params[:search],
+                                default_project_filter: default_project_filter)
     end
 
     def projects

app/services/search/group_service.rb

@@ -5,6 +5,7 @@ module Search
     def initialize(user, group, params)
       super(user, params)
 
+      @default_project_filter = false
       @group = group
     end
 

changelogs/unreleased/15955-improve-search-query.yml

+---
+title: Improve search query for merge requests.
+merge_request:
+author:
+type: performance

lib/gitlab/search_results.rb

@@ -27,10 +27,17 @@ module Gitlab
     # It allows us to search only for projects user has access to
     attr_reader :limit_projects
 
-    def initialize(current_user, limit_projects, query)
+    # Whether a custom filter is used to restrict scope of projects.
+    # If the default filter (which lists all projects user has access to)
+    # is used, we can skip it when filtering merge requests and optimize the
+    # query
+    attr_reader :default_project_filter
+
+    def initialize(current_user, limit_projects, query, default_project_filter: false)
       @current_user = current_user
       @limit_projects = limit_projects || Project.all
       @query = query
+      @default_project_filter = default_project_filter
     end
 
     def objects(scope, page = nil)
@@ -94,7 +101,11 @@ module Gitlab
     end
 
     def merge_requests
-      merge_requests = MergeRequestsFinder.new(current_user).execute.in_projects(project_ids_relation)
+      merge_requests = MergeRequestsFinder.new(current_user).execute
+      unless default_project_filter
+        merge_requests = merge_requests.in_projects(project_ids_relation)
+      end
+
       merge_requests =
         if query =~ /[#!](\d+)\z/
           merge_requests.where(iid: $1)

spec/lib/gitlab/search_results_spec.rb

@@ -51,6 +51,17 @@ describe Gitlab::SearchResults do
 
       expect(results.objects('merge_requests')).to include merge_request_2
     end
+
+    it 'includes project filter by default' do
+      expect(results).to receive(:project_ids_relation).and_call_original
+      results.objects('merge_requests')
+    end
+
+    it 'it skips project filter if default is used' do
+      allow(results).to receive(:default_project_filter).and_return(true)
+      expect(results).not_to receive(:project_ids_relation)
+      results.objects('merge_requests')
+    end
   end
 
   it 'does not list issues on private projects' do