reuse chunk to reduce memory usage in union scan (#7717)
reuse chunk to reduce memory usage in union scan (#7717)


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