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

1 files changed, 13 insertions, 2 deletions

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)