[opt](nereids)add explanation why we always update col stats in Stats…

…Calculator.

fe/fe-core/src/main/java/org/apache/doris/nereids/stats/StatsCalculator.java

@@ -227,6 +227,17 @@ private void estimate() {
             groupExpression.getOwnerGroup().setStatistics(newStats);
             groupExpression.setEstOutputRowCount(newStats.getRowCount());
         } else {
+            // the reason why we update col stats here.
+            // consider join between 3 tables: A/B/C with join condition: A.id=B.id=C.id and a filter: C.id=1
+            // in the final join result, the ndv of A.id/B.id/C.id should be 1
+            // suppose we have 2 candidate plans
+            // plan1: (A join B on A.id=B.id) join C on B.id=C.id
+            // plan2:(B join C)join A
+            // suppose plan1 is estimated before plan2
+            //
+            // after estimate the outer join of plan1 (join C), we update B.id.ndv=1, but A.id.ndv is not updated
+            // then we estimate plan2. the stats of plan2 is denoted by stats2. obviously, stats2.A.id.ndv is 1
+            // now we update OwnerGroup().getStatistics().A.id.ndv to 1
             groupExpression.getOwnerGroup().getStatistics().updateNdv(newStats);
         }
         groupExpression.setStatDerived(true);