Document how *_blks_read statistics columns are computed.

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9dfc0578d4a6b1f2557888d99254d05624e02a06..4b0c4999cd68c15c4ec50e09f08bd36437d0fecf 100644 (file)
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -927,8 +927,9 @@ postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <re
      <function>pg_stat_get_blocks_fetched</function> minus
      <function>pg_stat_get_blocks_hit</function> gives the number of kernel
      <function>read()</> calls issued for the table, index, or
-     database; but the actual number of physical reads is usually
-     lower due to kernel-level buffering.
+     database; the number of actual physical reads is usually
+     lower due to kernel-level buffering.  The <literal>*_blks_read</>
+     statistics columns uses this subtraction, i.e. fetched minus hit.
     </para>
    </note>