From: Magnus Hagander <magnus@hagander.net>
Date: Sun, 13 May 2007 16:04:22 +0000 (+0000)
Subject: Document that CLUSTER breaks MVCC visibility rules.
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=c54d0e49b44f4867f5757631d6e0b55881e67063;p=users%2Fbernd%2Fpostgres.git

Document that CLUSTER breaks MVCC visibility rules.
(Not needed in cvs head, because CLUSTER itself is fixed there)

Heikki Linnakangas
---

diff --git a/doc/src/sgml/ref/cluster.sgml b/doc/src/sgml/ref/cluster.sgml
index 1832693033..f57ec4a0da 100644
--- a/doc/src/sgml/ref/cluster.sgml
+++ b/doc/src/sgml/ref/cluster.sgml
@@ -99,6 +99,17 @@ CLUSTER
  <refsect1>
   <title>Notes</title>
 
+   <para>
+    <command>CLUSTER</command> loses all visibility information of tuples,
+    which makes the table look empty to any snapshot that was taken
+    before the <command>CLUSTER</command> command finished. That makes
+    <command>CLUSTER</command> unsuitable for applications where
+    transactions that access the table being clustered are run concurrently
+    with <command>CLUSTER</command>. This is most visible with serializable
+    transactions, because they take only one snapshot at the beginning of the
+    transaction, but read-committed transactions are also affected.
+   </para>
+
    <para>
     In cases where you are accessing single rows randomly
     within a table, the actual order of the data in the