diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 061697b..cf0a071 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2011,6 +2011,22 @@ include_dir 'conf.d'
          be inefficient.  The default value is 2.  Setting this value to 0
          disables parallel query execution.
         </para>
+
+        <para>
+         Note that parallel queries may consume very substantially more
+         resources than non-parallel queries, because each worker process is
+         a completely separate process which has roughly the same impact on the
+         system as an additional user session.  This should be taken into
+         account when choosing a value for this setting, as well as when
+         configuring other settings that control resource utilization, such
+         as <xref linkend="guc-work-mem">.  Resource limits such as
+         <varname>work_mem</> are applied individually to each worker,
+         which means the total utilization may be much higher across all
+         processes than it would normally be for any single process.
+         For example, a parallel query using 4 workers may use up to 5 times
+         as much CPU time, memory, I/O bandwidth, and so forth as a query which
+         uses no workers at all.
+        </para>
        </listitem>
       </varlistentry>
 
