From 272783be20927148d86b05a7e22f88b291c6dd93 Mon Sep 17 00:00:00 2001
From: Stan Hu <stanhu@gmail.com>
Date: Thu, 15 Mar 2018 13:12:00 -0700
Subject: [PATCH] Cache table_exists?('application_settings') to reduce
 repeated schema reloads

Closes #43355
---
 changelogs/unreleased/sh-cache-table-exists.yml |  5 +++++
 lib/gitlab/current_settings.rb                  |  2 +-
 lib/gitlab/database.rb                          |  5 +++++
 spec/lib/gitlab/database_spec.rb                | 12 ++++++++++++
 4 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 changelogs/unreleased/sh-cache-table-exists.yml

diff --git a/changelogs/unreleased/sh-cache-table-exists.yml b/changelogs/unreleased/sh-cache-table-exists.yml
new file mode 100644
index 00000000000..37407b2a005
--- /dev/null
+++ b/changelogs/unreleased/sh-cache-table-exists.yml
@@ -0,0 +1,5 @@
+---
+title: Cache table_exists?('application_settings') to reduce repeated schema reloads
+merge_request:
+author:
+type: performance
diff --git a/lib/gitlab/current_settings.rb b/lib/gitlab/current_settings.rb
index b7c596a973d..e392a015b91 100644
--- a/lib/gitlab/current_settings.rb
+++ b/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
diff --git a/lib/gitlab/database.rb b/lib/gitlab/database.rb
index d4fc69cb173..76501dd50e8 100644
--- a/lib/gitlab/database.rb
+++ b/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
diff --git a/spec/lib/gitlab/database_spec.rb b/spec/lib/gitlab/database_spec.rb
index 689bbbfaa8d..1fe1d3926ad 100644
--- a/spec/lib/gitlab/database_spec.rb
+++ b/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)