Clarify vacuum lazy can shrink a file under certain circumstances.

diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 3854964525157088dc26a0c3195c45ee16132685..26ef633d7a88021b7cc992aac274d19d7d40a16d 100644 (file)
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -141,11 +141,12 @@ $PostgreSQL$
     command. The first form, known as <quote>lazy vacuum</quote> or
     just <command>VACUUM</command>, marks expired data in tables and
     indexes for future reuse; it does <emphasis>not</emphasis> attempt
-    to reclaim the space used by this expired data
-    immediately. Therefore, the table file is not shortened, and any
-    unused space in the file is not returned to the operating
-    system. This variant of <command>VACUUM</command> can be run
-    concurrently with normal database operations.
+    to reclaim the space used by this expired data unless the space is
+    at the end of the table and an exclusive lock can be obtained easily
+    on the table. Unused space at the start or middle of the file does
+    not result in the file being shortened and space returned to the
+    operating system. This variant of <command>VACUUM</command> can be
+    run concurrently with normal database operations.
    </para>
 
    <para>