docs -clarify  GUC default_transaction_deferrable has not effect.
<topic id="default_transaction_deferrable">
<title>default_transaction_deferrable</title>
<body>
- <note>Only read-only, serializable transactions can be deferred. Greenplum Database does not
- support the <codeph>SERIALIZABLE</codeph> transaction isolation level, so setting
- <codeph>default_transaction_deferrable</codeph> to on has no effect in Greenplum
- Database.</note>
- <p>When running at the serializable isolation level, a deferrable read-only SQL transaction
- may be delayed before it is allowed to proceed. However, once it begins executing it does
- not incur any of the overhead required to ensure serializability; so serialization code will
- have no reason to force it to abort because of concurrent updates, making this option
- suitable for long-running read-only transactions.</p>
+ <p>When running at the <codeph>SERIALIZABLE</codeph> isolation level, a deferrable read-only
+ SQL transaction may be delayed before it is allowed to proceed. However, once it begins
+ executing it does not incur any of the overhead required to ensure serializability; so
+ serialization code will have no reason to force it to abort because of concurrent updates,
+ making this option suitable for long-running read-only transactions.</p>
<p>This parameter controls the default deferrable status of each new transaction. It currently
has no effect on read-write transactions or those operating at isolation levels lower than
- serializable. The default is off.</p>
+ <codeph>SERIALIZABLE</codeph>. The default is <codeph>off</codeph>.
+ <note>Setting <codeph>default_transaction_deferrable</codeph> to <codeph>on</codeph> has no
+ effect in Greenplum Database. Only read-only, <codeph>SERIALIZABLE</codeph> transactions
+ can be deferred. However, Greenplum Database does not support the
+ <codeph>SERIALIZABLE</codeph> transaction isolation level. See <xref
+ href="../sql_commands/SET_TRANSACTION.xml"/>.</note></p>
<table id="default_transaction_deferrable">
<tgroup cols="3">
<colspec colnum="1" colname="col1" colwidth="1*"/>
