From 71f996d22125eb6cfdbee6094f44370aa8dec610 Mon Sep 17 00:00:00 2001
From: Andrew Dunstan <andrew@dunslane.net>
Date: Tue, 27 Dec 2016 11:23:46 -0500
Subject: [PATCH] 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.
---
doc/src/sgml/pgstattuple.sgml | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/doc/src/sgml/pgstattuple.sgml b/doc/src/sgml/pgstattuple.sgml
index 9ada5d209af..d2fa524d6ec 100644
--- a/doc/src/sgml/pgstattuple.sgml
+++ b/doc/src/sgml/pgstattuple.sgml
@@ -117,6 +117,16 @@ free_percent | 1.95
</tgroup>
</table>
+ <note>
+ <para>
+ The <literal>table_len</literal> will always be greater than the sum
+ of the <literal>tuple_len</literal>, <literal>dead_tuple_len</literal>
+ and <literal>free_space</literal>. 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.
+ </para>
+ </note>
+
<para>
<function>pgstattuple</function> acquires only a read lock on the
relation. So the results do not reflect an instantaneous snapshot;
--
GitLab