Minor optimization in tuplesort_skiptuples_mk for in memory non unique tuple store which just advances current position by N instead of getting tuple N times.
Minor optimization in tuplesort_skiptuples_mk for in memory non unique
tuple store which just advances current position by N instead of getting
tuple N times.

Author: Max Yang <121e2a1f9ac7c93bff05080563bf586498521b0d@pivotal.io>

Chibin/gpdb,greenplum-db/gpdb,Chibin/gpdb,xinzweb/gpdb,Chibin/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,Chibin/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,Chibin/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,Chibin/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,Chibin/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,Chibin/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb