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authorAndrew Dunstan <andrew@dunslane.net>2016-12-27 11:23:46 -0500
committerAndrew Dunstan <andrew@dunslane.net>2016-12-27 11:23:46 -0500
commit71f996d22125eb6cfdbee6094f44370aa8dec610 (patch)
treef2a805eafe131d605898d7b52684c39e847443b4
parent3ea56fffd66fe2c5857f36359ae16d4cabcdeef6 (diff)
downloadpostgresql-71f996d22125eb6cfdbee6094f44370aa8dec610.tar.gz
postgresql-71f996d22125eb6cfdbee6094f44370aa8dec610.zip
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.
-rw-r--r--doc/src/sgml/pgstattuple.sgml10
1 files changed, 10 insertions, 0 deletions
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;