Merge branch 'ab/select1-empty' into 'master'

Use empty query instead SELECT 1

See merge request gitlab-org/gitlab!49730

config/initializers/active_record_ping.rb

+# # frozen_string_literal: true
+
+if Gitlab::Utils.to_boolean(ENV['ENABLE_ACTIVERECORD_EMPTY_PING'], default: false)
+  ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.prepend(Gitlab::Database::PostgresqlAdapter::EmptyQueryPing)
+end

lib/gitlab/database/postgresql_adapter/empty_query_ping.rb

+# frozen_string_literal: true
+
+# rubocop:disable Gitlab/ModuleWithInstanceVariables
+module Gitlab
+  module Database
+    module PostgresqlAdapter
+      module EmptyQueryPing
+        # ActiveRecord uses `SELECT 1` to check if the connection is alive
+        # We patch this here to use an empty query instead, which is a bit faster
+        def active?
+          @lock.synchronize do
+            @connection.query ';'
+          end
+          true
+        rescue PG::Error
+          false
+        end
+      end
+    end
+  end
+end
+# rubocop:enable Gitlab/ModuleWithInstanceVariables

spec/lib/gitlab/database/postgresql_adapter/empty_query_ping_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::PostgresqlAdapter::EmptyQueryPing do
+  describe '#active?' do
+    let(:adapter_class) do
+      Class.new do
+        include Gitlab::Database::PostgresqlAdapter::EmptyQueryPing
+
+        def initialize(connection, lock)
+          @connection = connection
+          @lock = lock
+        end
+      end
+    end
+
+    subject { adapter_class.new(connection, lock).active? }
+
+    let(:connection) { double(query: nil) }
+    let(:lock) { double }
+
+    before do
+      allow(lock).to receive(:synchronize).and_yield
+    end
+
+    it 'uses an empty query to check liveness' do
+      expect(connection).to receive(:query).with(';')
+
+      subject
+    end
+
+    it 'returns true if no error was signaled' do
+      expect(subject).to be_truthy
+    end
+
+    it 'returns false when an error occurs' do
+      expect(lock).to receive(:synchronize).and_raise(PG::Error)
+
+      expect(subject).to be_falsey
+    end
+  end
+end