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author | Andrew Dunstan <andrew@dunslane.net> | 2016-12-27 11:23:46 -0500 |
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committer | Andrew Dunstan <andrew@dunslane.net> | 2016-12-27 11:23:46 -0500 |
commit | 71f996d22125eb6cfdbee6094f44370aa8dec610 (patch) | |
tree | f2a805eafe131d605898d7b52684c39e847443b4 | |
parent | 3ea56fffd66fe2c5857f36359ae16d4cabcdeef6 (diff) | |
download | postgresql-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.sgml | 10 |
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; |