doc: Add more details about pg_stat_get_xact_blocks_{fetched,hit}

The explanation describing the dependency to system read() calls for
these two functions has been removed in ddfc2d9.  And after more
discussion about d69c404, we have concluded that adding more details
makes them easier to understand.

While on it, use the term "block read requests" (maybe found in cache)
rather than "buffers fetched" and "buffer hits".

Per discussion with Melanie Plageman, Kyotaro Horiguchi, Bertrand
Drouvot and myself.

Discussion: https://postgr.es/m/CAAKRu_ZmdiScT4q83OAbfmR5AH-L5zWya3SXjaxiJvhCob-e2A@mail.gmail.com
Backpatch-through: 11

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 65da082ed9bbc42eb2d701395a4b7ebfeb7ff593..fd0ffbb1e08e79aba870d09baaf6fc2c88c1ef5d 100644 (file)
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -5735,8 +5735,11 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
         <returnvalue>bigint</returnvalue>
        </para>
        <para>
-        Returns the number of buffers fetched for table or index, in the current
-        transaction.
+        Returns the number of block read requests for table or index, in the
+        current transaction. This number minus
+        <function>pg_stat_get_xact_blocks_hit</function> gives the number of
+        kernel <function>read()</function> calls; the number of actual
+        physical reads is usually lower due to kernel-level buffering.
        </para></entry>
       </row>
 
@@ -5749,8 +5752,9 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
         <returnvalue>bigint</returnvalue>
        </para>
        <para>
-        Returns the number of buffer hits for table or index, in the current
-        transaction.
+        Returns the number of block read requests for table or index, in the
+        current transaction, found in cache (not triggering kernel
+        <function>read()</function> calls).
        </para></entry>
       </row>