Improve label search with issuable optimization

This commit improves the label lookup queries when the
`optimized_issuable_label_filter` feature flag is enabled.

app/models/concerns/optimized_issuable_label_filter.rb

 # frozen_string_literal: true
 
 module OptimizedIssuableLabelFilter
+  extend ActiveSupport::Concern
+
+  prepended do
+    extend Gitlab::Cache::RequestCache
+
+    # Avoid repeating label queries times when the finder is instantiated multiple times during the request.
+    request_cache(:find_label_ids) { [root_namespace.id, params.label_names] }
+  end
+
   def by_label(items)
     return items unless params.labels?
 
@@ -41,7 +50,7 @@ module OptimizedIssuableLabelFilter
 
   def issuables_with_selected_labels(items, target_model)
     if root_namespace
-      all_label_ids = find_label_ids(root_namespace)
+      all_label_ids = find_label_ids
       # Found less labels in the DB than we were searching for. Return nothing.
       return items.none if all_label_ids.size != params.label_names.size
 
@@ -57,18 +66,20 @@ module OptimizedIssuableLabelFilter
     items
   end
 
-  def find_label_ids(root_namespace)
-    finder_params = {
-      include_subgroups: true,
-      include_ancestor_groups: true,
-      include_descendant_groups: true,
-      group: root_namespace,
-      title: params.label_names
-    }
-
-    LabelsFinder
-      .new(nil, finder_params)
-      .execute(skip_authorization: true)
+  def find_label_ids
+    group_labels = Label
+      .where(project_id: nil)
+      .where(title: params.label_names)
+      .where(group_id: root_namespace.self_and_descendants.select(:id))
+
+    project_labels = Label
+      .where(group_id: nil)
+      .where(title: params.label_names)
+      .where(project_id: Project.select(:id).where(namespace_id: root_namespace.self_and_descendants.select(:id)))
+
+    Label
+      .from_union([group_labels, project_labels], remove_duplicates: false)
+      .reorder(nil)
       .pluck(:title, :id)
       .group_by(&:first)
       .values

changelogs/unreleased/speed-up-label-query-for-optimized-issuable-finders.yml

+---
+title: Replace wrong index definition on labels (project_id, title)
+merge_request: 48238
+author:
+type: other

db/migrate/20201120125953_replace_unused_labels_index.rb

+# frozen_string_literal: true
+
+class ReplaceUnusedLabelsIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  NEW_INDEX_NAME = 'index_labels_on_group_id_and_title_with_null_project_id'
+  OLD_INDEX_NAME = 'index_labels_on_group_id_and_title'
+
+  def up
+    add_concurrent_index :labels, [:group_id, :title], where: 'project_id IS NULL', name: NEW_INDEX_NAME
+    remove_concurrent_index_by_name :labels, OLD_INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :labels, [:group_id, :title], where: 'project_id = NULL::integer', name: OLD_INDEX_NAME
+    remove_concurrent_index_by_name :labels, NEW_INDEX_NAME
+  end
+end

db/schema_migrations/20201120125953

+8b60a6bc892f9700df81de9909595544f9f820621a210906a249428ddec9eefa
\ No newline at end of file

db/structure.sql

@@ -21202,7 +21202,7 @@ CREATE UNIQUE INDEX index_label_priorities_on_project_id_and_label_id ON label_p
 
 CREATE UNIQUE INDEX index_labels_on_group_id_and_project_id_and_title ON labels USING btree (group_id, project_id, title);
 
-CREATE INDEX index_labels_on_group_id_and_title ON labels USING btree (group_id, title) WHERE (project_id = NULL::integer);
+CREATE INDEX index_labels_on_group_id_and_title_with_null_project_id ON labels USING btree (group_id, title) WHERE (project_id IS NULL);
 
 CREATE INDEX index_labels_on_project_id ON labels USING btree (project_id);