From 56a6317bf54625c7fdade6cd1ab38178bba42448 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Thu, 24 Jan 2013 21:44:54 -0500
Subject: [PATCH] doc: add mention of ssi read anomolies to mvcc docs
From Jeff Davis, modified by Kevin Grittner
---
doc/src/sgml/mvcc.sgml | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)
diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index d5c6076d4aa..db820d6f43f 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -515,8 +515,9 @@ ERROR: could not serialize access due to concurrent update
</indexterm>
<para>
- The <firstterm>Serializable</firstterm> isolation level provides the strictest transaction
- isolation. This level emulates serial transaction execution,
+ The <firstterm>Serializable</firstterm> isolation level provides
+ the strictest transaction isolation. This level emulates serial
+ transaction execution for all committed transactions;
as if transactions had been executed one after another, serially,
rather than concurrently. However, like the Repeatable Read level,
applications using this level must
@@ -571,6 +572,20 @@ ERROR: could not serialize access due to read/write dependencies among transact
computed by A.
</para>
+ <para>
+ When relying on Serializable transactions to prevent anomalies, it is
+ important that any data read from a permanent user table not be
+ considered valid until the transaction which read it has successfully
+ committed. This is true even for read-only transactions, except that
+ data read within a <firstterm>deferrable</firstterm> read-only
+ transaction is known to be valid as soon as it is read, because such a
+ transaction waits until it can acquire a snapshot guaranteed to be free
+ from such problems before starting to read any data. In all other cases
+ applications must not depend on results read during a transaction that
+ later aborted; instead, they should retry the transaction until it
+ succeeds.
+ </para>
+
<para>
To guarantee true serializability <productname>PostgreSQL</productname>
uses <firstterm>predicate locking</>, which means that it keeps locks
--
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