Recommend ALTER TABLE ... TYPE as the best way to reclaim space occupied by deleted...

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 6119a150626c1123a1c0bd41440699c1b7c0384d..e86712e26b5e46a2c4808306b4901057c7df17d3 100644 (file)
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -1,5 +1,5 @@
 
 
@@ -399,17 +399,26 @@ where action is one of:
    
     The DROP COLUMN form does not physically remove
     the column, but simply makes it invisible to SQL operations.  Subsequent
-    insert and update operations in the table will store a null value for the column.
-    Thus, dropping a column is quick but it will not immediately reduce the
-    on-disk size of your table, as the space occupied 
+    insert and update operations in the table will store a null value for the
+    column. Thus, dropping a column is quick but it will not immediately
+    reduce the on-disk size of your table, as the space occupied 
     by the dropped column is not reclaimed.  The space will be
     reclaimed over time as existing rows are updated.
-    To reclaim the space at once, do a dummy UPDATE of all rows
-    and then vacuum, as in:
+   
+
+   
+    The fact that ALTER TYPE requires rewriting the whole table
+    is sometimes an advantage, because the rewriting process eliminates
+    any dead space in the table.  For example, to reclaim the space occupied
+    by a dropped column immediately, the fastest way is
 
-UPDATE table SET col = col;
-VACUUM FULL table;
+ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
 
+    where anycol is any remaining table column and
+    anytype is the same type that column already has.
+    This results in no semantically-visible change in the table,
+    but the command forces rewriting, which gets rid of no-longer-useful
+    data.