Added using index option to cluster
<topic id="topic1"><title id="bh20941">CLUSTER</title><body><p id="sql_command_desc">Physically reorders a heap storage table on disk according to an index.
Not a recommended operation in Greenplum Database.</p><section id="section2"><title>Synopsis</title><codeblock id="sql_command_synopsis">CLUSTER <varname>indexname</varname> ON <varname>tablename</varname>
-CLUSTER [VERBOSE] <varname>tablename</varname>
+CLUSTER [VERBOSE] <varname>tablename</varname> [ USING index_name ]
CLUSTER [VERBOSE]</codeblock></section><section id="section3"><title>Description</title><p><codeph>CLUSTER</codeph> orders a heap storage table based on an index.
<codeph>CLUSTER</codeph> on non-B-tree indexes is not supported on append-optmized storage
