diff --git a/nixos/doc/manual/release-notes/rl-1803.xml b/nixos/doc/manual/release-notes/rl-1803.xml
index 46cbeb0a158..8391c550afa 100644
--- a/nixos/doc/manual/release-notes/rl-1803.xml
+++ b/nixos/doc/manual/release-notes/rl-1803.xml
@@ -182,6 +182,20 @@ following incompatible changes:</para>
       <literal>lib.mkOverride</literal> can be used.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The following changes apply if the <literal>stateVersion</literal> is changed to 18.03 or higher.
+      For <literal>stateVersion = "17.09"</literal> or lower the old behavior is preserved.
+    </para>
+    <itemizedlist>
+      <listitem>
+        <para>
+          <literal>matrix-synapse</literal> uses postgresql by default instead of sqlite.
+          Migration instructions can be found <link xlink:href="https://github.com/matrix-org/synapse/blob/master/docs/postgres.rst#porting-from-sqlite"> here </link>.
+        </para>
+      </listitem>
+    </itemizedlist>
+  </listitem>
 </itemizedlist>
 
 </section>
diff --git a/nixos/modules/services/misc/matrix-synapse.nix b/nixos/modules/services/misc/matrix-synapse.nix
index 33b3b17359c..b153ebbc98c 100644
--- a/nixos/modules/services/misc/matrix-synapse.nix
+++ b/nixos/modules/services/misc/matrix-synapse.nix
@@ -341,7 +341,9 @@ in {
       };
       database_type = mkOption {
         type = types.enum [ "sqlite3" "psycopg2" ];
-        default = "sqlite3";
+        default = if versionAtLeast config.system.stateVersion "18.03"
+          then "psycopg2"
+          else "sqlite3";
         description = ''
           The database engine name. Can be sqlite or psycopg2.
         '';