Cache table_exists?('application_settings') to reduce repeated schema reloads

Closes #43355

changelogs/unreleased/sh-cache-table-exists.yml

+---
+title: Cache table_exists?('application_settings') to reduce repeated schema reloads
+merge_request:
+author:
+type: performance

lib/gitlab/current_settings.rb

@@ -70,7 +70,7 @@ module Gitlab
         active_db_connection = ActiveRecord::Base.connection.active? rescue false
 
         active_db_connection &&
-          ActiveRecord::Base.connection.table_exists?('application_settings')
+          Gitlab::Database.cached_table_exists?('application_settings')
       rescue ActiveRecord::NoDatabaseError
         false
       end

lib/gitlab/database.rb

@@ -187,6 +187,11 @@ module Gitlab
       connection.schema_cache.columns_hash(table_name).has_key?(column_name.to_s)
     end
 
+    def self.cached_table_exists?(table_name)
+      # Rails 5 uses data_source_exists? instead of table_exists?
+      connection.schema_cache.table_exists?(table_name)
+    end
+
     private_class_method :connection
 
     def self.database_version

spec/lib/gitlab/database_spec.rb

@@ -298,6 +298,18 @@ describe Gitlab::Database do
     end
   end
 
+  describe '.cached_table_exists?' do
+    it 'only retrieves data once per table' do
+      expect(ActiveRecord::Base.connection).to receive(:table_exists?).with(:projects).once.and_call_original
+      expect(ActiveRecord::Base.connection).to receive(:table_exists?).with(:bogus_table_name).once.and_call_original
+
+      2.times do
+        expect(described_class.cached_table_exists?(:projects)).to be_truthy
+        expect(described_class.cached_table_exists?(:bogus_table_name)).to be_falsey
+      end
+    end
+  end
+
   describe '#true_value' do
     it 'returns correct value for PostgreSQL' do
       expect(described_class).to receive(:postgresql?).and_return(true)