update comment

planner/core/rule_topn_push_down.go

@@ -137,13 +137,13 @@ func (p *LogicalProjection) pushDownTopN(topN *LogicalTopN, opt *logicalOptimize
 			}
 		}
 
-		// if the projection contains a column(with ID=0) in topN.ByItems, projection will prevent the optimizer from pushing topN down.
+		// if topN.ByItems contains a column(with ID=0) generated by projection, projection will prevent the optimizer from pushing topN down.
 		for _, by := range topN.ByItems {
 			cols := expression.ExtractColumns(by.Expr)
 			for _, col := range cols {
 				if col.ID == 0 && p.Schema().Contains(col) {
+					// check whether the column is generated by projection
 					if !p.children[0].Schema().Contains(col) {
-						// the column is generated by projection
 						p.children[0] = p.children[0].pushDownTopN(nil, opt)
 						return topN.setChild(p, opt)
 					}