diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
new file mode 100644
index 3a84321..19e3e36
*** a/doc/src/sgml/config.sgml
--- b/doc/src/sgml/config.sgml
*************** SET ENABLE_SEQSCAN TO OFF;
*** 2590,2595 ****
--- 2590,2597 ----
         <para>
          Sets the planner's estimate of the cost of a
          non-sequentially-fetched disk page.  The default is 4.0.
+         (The default is lower than the typical difference between random
+         and sequential storage access speed because of caching effects.)
          This value can be overridden for tables and indexes in a particular
          tablespace by setting the tablespace parameter of the same name
          (see <xref linkend="sql-altertablespace">).