tidb: add checkPlan function to check index plan is used.
tidb: add checkPlan function to check index plan is used.

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