Skip projects filter on merge requests searchjprovazn-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

5 files changed, 34 insertions, 3 deletions

diff --git a/app/services/search/global_service.rb b/app/services/search/global_service.rb
index ff188102b62..92a32e703af 100644
--- a/app/services/search/global_service.rb
+++ b/app/services/search/global_service.rb
@@ -1,13 +1,16 @@
 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
diff --git a/app/services/search/group_service.rb b/app/services/search/group_service.rb
index 29478e3251f..b4efba68715 100644
--- a/app/services/search/group_service.rb
+++ b/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
 
diff --git a/changelogs/unreleased/15955-improve-search-query.yml b/changelogs/unreleased/15955-improve-search-query.yml
new file mode 100644
index 00000000000..80cb8af617f
--- /dev/null
+++ b/changelogs/unreleased/15955-improve-search-query.yml
@@ -0,0 +1,5 @@
+---
+title: Improve search query for merge requests.
+merge_request:
+author:
+type: performance
diff --git a/lib/gitlab/search_results.rb b/lib/gitlab/search_results.rb
index fef9d3e31d4..7037e2e61cc 100644
--- a/lib/gitlab/search_results.rb
+++ b/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)
diff --git a/spec/lib/gitlab/search_results_spec.rb b/spec/lib/gitlab/search_results_spec.rb
index e44a7c23452..a958baab44f 100644
--- a/spec/lib/gitlab/search_results_spec.rb
+++ b/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