Cache column_exists? for Elasticsearch columns

Any time a searchable object (e.g. issue, merge request, project, etc.)
is updated, the `after_commit` hook checks whetether two Elasticsearch
columns are present in the application_settings table. This leads to
unnecessary queries in production as the value can just be cached by
connection.

Closes gitlab-org/gitlab-ce#43355

ee/app/models/ee/application_setting.rb

@@ -133,11 +133,11 @@ module EE
     end
 
     def elasticsearch_indexing_column_exists?
-      ActiveRecord::Base.connection.column_exists?(:application_settings, :elasticsearch_indexing)
+      ::Gitlab::Database.cached_column_exists?(:application_settings, :elasticsearch_indexing)
     end
 
     def elasticsearch_search_column_exists?
-      ActiveRecord::Base.connection.column_exists?(:application_settings, :elasticsearch_search)
+      ::Gitlab::Database.cached_column_exists?(:application_settings, :elasticsearch_search)
     end
   end
 end

ee/changelogs/unreleased/sh-cache-column-exists-cache.yml

+---
+title: Cache column_exists? for Elasticsearch columns
+merge_request:
+author:
+type: performance

lib/gitlab/database.rb

@@ -199,6 +199,10 @@ module Gitlab
       ActiveRecord::Base.connection
     end
 
+    def self.cached_column_exists?(table_name, column_name)
+      connection.schema_cache.columns_hash(table_name).has_key?(column_name.to_s)
+    end
+
     private_class_method :connection
 
     def self.database_version

spec/lib/gitlab/database_spec.rb

@@ -287,6 +287,17 @@ describe Gitlab::Database do
     end
   end
 
+  describe '.cached_column_exists?' do
+    it 'only retrieves data once' do
+      expect(ActiveRecord::Base.connection).to receive(:columns).once.and_call_original
+
+      2.times do
+        expect(described_class.cached_column_exists?(:projects, :id)).to be_truthy
+        expect(described_class.cached_column_exists?(:projects, :bogus_column)).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)