Let's initialize instance variables in the postgres adapter.

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -218,6 +218,9 @@ module ActiveRecord
 
         # @local_tz is initialized as nil to avoid warnings when connect tries to use it
         @local_tz = nil
+        @table_alias_length = nil
+        @postgresql_version = nil
+
         connect
         @local_tz = execute('SHOW TIME ZONE').first["TimeZone"]
       end

activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb

@@ -0,0 +1,17 @@
+require "cases/helper"
+
+module ActiveRecord
+  module ConnectionAdapters
+    class PostgreSQLAdapterTest < ActiveRecord::TestCase
+      def setup
+        @connection = ActiveRecord::Base.connection
+      end
+
+      def test_table_alias_length
+        assert_nothing_raised do
+          @connection.table_alias_length
+        end
+      end
+    end
+  end
+end