executor: improve performance of aggregation which have no group-by item. (#6210)
executor: improve performance of aggregation which have no group-by item. (#6210)


ngaut/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,tiancaiamao/tidb,c4pt0r/tidb,CodeRushing/tidb,cosmtrek/tidb,pingcap/tidb,cosmtrek/tidb,kyle-wang/tidb,XuHuaiyu/tidb,kyle-wang/tidb,qiuyesuifeng/tidb,coocood/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,mccxj/tidb,c4pt0r/tidb,qiuyesuifeng/tidb,CodeRushing/tidb,ming-relax/tidb,pingcap/tidb,wusphinx/tidb,c4pt0r/tidb,c4pt0r/tidb,pingcap/tidb,wusphinx/tidb,ngaut/tidb,mccxj/tidb,EvilMcJerkface/tidb,Nify/tidb,tiancaiamao/tidb,Cofyc/tidb,kyle-wang/tidb,ming-relax/tidb,Nify/tidb,ngaut/tidb,pingcap/tidb,c4pt0r/tidb,EvilMcJerkface/tidb,XuHuaiyu/tidb,Cofyc/tidb,kyle-wang/tidb,coocood/tidb,ngaut/tidb,kyle-wang/tidb