Replace marked_for_deletion_at column with id to get index only scan

db/post_migrate/20211203091642_add_index_to_projects_on_marked_for_deletion_at.rb

@@ -6,10 +6,10 @@ class AddIndexToProjectsOnMarkedForDeletionAt < Gitlab::Database::Migration[1.0]
   INDEX_NAME = 'index_projects_not_aimed_for_deletion'
 
   def up
-    add_concurrent_index :projects, :marked_for_deletion_at, where: 'marked_for_deletion_at IS NULL', name: INDEX_NAME
+    add_concurrent_index :projects, :id, where: 'marked_for_deletion_at IS NULL', name: INDEX_NAME
   end
 
   def down
-    remove_concurrent_index :projects, :marked_for_deletion_at, name: INDEX_NAME
+    remove_concurrent_index :projects, :id, name: INDEX_NAME
   end
 end

db/structure.sql

@@ -27108,7 +27108,7 @@ CREATE INDEX index_projects_api_vis20_path ON projects USING btree (path, id) WH
 
 CREATE INDEX index_projects_api_vis20_updated_at ON projects USING btree (updated_at, id) WHERE (visibility_level = 20);
 
-CREATE INDEX index_projects_not_aimed_for_deletion ON projects USING btree (marked_for_deletion_at) WHERE (marked_for_deletion_at IS NULL);
+CREATE INDEX index_projects_not_aimed_for_deletion ON projects USING btree (id) WHERE (marked_for_deletion_at IS NULL);
 
 CREATE INDEX index_projects_on_created_at_and_id ON projects USING btree (created_at, id);