[revert](nereids) Revert data size agg (#21216)

To make stats derivation more precise

fe/fe-core/src/main/java/org/apache/doris/statistics/BaseAnalysisTask.java

@@ -182,7 +182,10 @@ public abstract class BaseAnalysisTask {
 
     // TODO : time cost is intolerable when column is string type, return 0 directly for now.
     protected String getDataSizeFunction(Column column) {
-        return "0";
+        if (column.getType().isStringType()) {
+            return "SUM(LENGTH(`${colName}`))";
+        }
+        return "COUNT(1) * " + column.getType().getSlotSize();
     }
 
     private boolean isUnsupportedType(PrimitiveType type) {