Improve snippet finders query

app/finders/snippets_finder.rb

@@ -117,7 +117,7 @@ class SnippetsFinder < UnionFinder
       queries << snippets_of_authorized_projects if current_user
     end
 
-    find_union(queries, Snippet)
+    prepared_union(queries)
   end
 
   def snippets_for_a_single_project
@@ -208,6 +208,17 @@ class SnippetsFinder < UnionFinder
   def sort_param
     sort.presence || 'id_desc'
   end
+
+  def prepared_union(queries)
+    return Snippet.none if queries.empty?
+    return queries.first if queries.length == 1
+
+    # The queries are going to be part of a global `where`
+    # therefore we only need to retrieve the `id` column
+    # which will speed the query
+    queries.map! { |rel| rel.select(:id) }
+    Snippet.id_in(find_union(queries, Snippet))
+  end
 end
 
 SnippetsFinder.prepend_if_ee('EE::SnippetsFinder')

changelogs/unreleased/fj-227311-optimize-snippets-finder-query.yml

+---
+title: Improve snippet finders queries
+merge_request: 36292
+author:
+type: performance

ee/app/finders/ee/snippets_finder.rb

@@ -34,7 +34,7 @@ module EE
         queries << snippets_of_authorized_projects unless only_personal
       end
 
-      find_union(queries, ::Snippet)
+      prepared_union(queries)
     end
 
     def restricted_personal_snippets

ee/spec/finders/snippets_finder_spec.rb

@@ -181,6 +181,14 @@ RSpec.describe SnippetsFinder do
             private_project_snippet
           )
       end
+
+      context 'when no personal snippets are visible' do
+        let(:user) { nil }
+
+        it 'does not return any record' do
+          expect(subject).to be_empty
+        end
+      end
     end
   end
 end

spec/finders/snippets_finder_spec.rb

@@ -283,6 +283,12 @@ RSpec.describe SnippetsFinder do
       it 'returns only personal snippets when the user cannot read cross project' do
         expect(described_class.new(user).execute).to contain_exactly(private_personal_snippet, internal_personal_snippet, public_personal_snippet)
       end
+
+      context 'when only project snippets are required' do
+        it 'returns no records' do
+          expect(described_class.new(user, only_project: true).execute).to be_empty
+        end
+      end
     end
 
     context 'when project snippets are disabled' do