expression: create a new aggregate distinct if not.
expression: create a new aggregate distinct if not.

Fix #324

myrice/tidb,pingcap/tidb,liyu4/tidb,c4pt0r/tidb,astaxie/tidb,liyu4/tidb,ngaut/tidb,cosmtrek/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,CodeRushing/tidb,EvilMcJerkface/tidb,8cbx/tidb,silenceper/tidb,ngaut/tidb,c4pt0r/tidb,ueizhou/tidb,XuHuaiyu/tidb,zimulala/tidb,netroby/tidb,andelf/tidb,astaxie/tidb,wusphinx/tidb,kyle-wang/tidb,kyle-wang/tidb,qiuyesuifeng/tidb,coocood/tidb,qhsong/tidb,spongedu/tidb,ueizhou/tidb,8cbx/tidb,netroby/tidb,spongedu/tidb,coocood/tidb,andelf/tidb,ngaut/tidb,zs634134578/tidb,ming-relax/tidb,qiuyesuifeng/tidb,mccxj/tidb,pingcap/tidb,cosmtrek/tidb,c4pt0r/tidb,JinheLin/tidb,myrice/tidb,yuanwhy/tidb,zs634134578/tidb,anywhy/tidb,silentred/tidb,gaocegege/tidb,louishust/tidb,wusphinx/tidb,EvilMcJerkface/tidb,zimulala/tidb,kyle-wang/tidb,louishust/tidb,XuHuaiyu/tidb,pingcap/tidb,tiancaiamao/tidb,Nify/tidb,mccxj/tidb,qhsong/tidb,pingcap/tidb,CodeRushing/tidb,c4pt0r/tidb,Cofyc/tidb,ngaut/tidb,simon-xia/tidb,silenceper/tidb,yangwenmai/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,JinheLin/tidb,gaocegege/tidb,yangwenmai/tidb,tiancaiamao/tidb,yuanwhy/tidb,pingcap/tidb,simon-xia/tidb,ming-relax/tidb,pingcap/tidb,anywhy/tidb,silentred/tidb,kyle-wang/tidb,Cofyc/tidb,Nify/tidb