Memoize Gitlab::Database.version

This removes the need for running a database query every time we want to
check the database version.

changelogs/unreleased/memoize-database-version.yml

+---
+title: Memoize Gitlab::Database.version
+merge_request:
+author:
+type: performance

lib/gitlab/database.rb

@@ -43,7 +43,7 @@ module Gitlab
     end
 
     def self.version
-      database_version.match(/\A(?:PostgreSQL |)([^\s]+).*\z/)[1]
+      @version ||= database_version.match(/\A(?:PostgreSQL |)([^\s]+).*\z/)[1]
     end
 
     def self.join_lateral_supported?

spec/lib/gitlab/database_spec.rb

@@ -32,6 +32,12 @@ describe Gitlab::Database do
   end
 
   describe '.version' do
+    around do |example|
+      described_class.instance_variable_set(:@version, nil)
+      example.run
+      described_class.instance_variable_set(:@version, nil)
+    end
+
     context "on mysql" do
       it "extracts the version number" do
         allow(described_class).to receive(:database_version)
@@ -49,6 +55,14 @@ describe Gitlab::Database do
         expect(described_class.version).to eq '9.4.4'
       end
     end
+
+    it 'memoizes the result' do
+      count = ActiveRecord::QueryRecorder
+        .new { 2.times { described_class.version } }
+        .count
+
+      expect(count).to eq(1)
+    end
   end
 
   describe '.join_lateral_supported?' do