Explain unaccounted for space in pgstattuple.

In addition to space accounted for by tuple_len, dead_tuple_len and
free_space, the table_len includes page overhead, the item pointers
table and padding bytes.

Backpatch to live branches.

diff --git a/doc/src/sgml/pgstattuple.sgml b/doc/src/sgml/pgstattuple.sgml
index 61340bedbc37866be92abc5558c4a7a7150dbb45..4a220992ce99d318d92c3faace056894496c2056 100644 (file)
--- a/doc/src/sgml/pgstattuple.sgml
+++ b/doc/src/sgml/pgstattuple.sgml
@@ -109,6 +109,16 @@ free_percent       | 1.95
      
     
 
+    
+      
+        The table_len will always be greater than the sum
+        of the tuple_len, dead_tuple_len
+        and free_space. The difference is accounted for by
+        fixed page overhead, the per-page table of pointers to tuples, and
+        padding to ensure that tuples are correctly aligned. 
+      
+    
+
     
      pgstattuple acquires only a read lock on the
      relation. So the results do not reflect an instantaneous snapshot;