[Fix](Planner) fix limit execute before sort in show export job (#21663)

Problem:
When doing show export jobs, limit would execute before sort before changed. So the result would not be expected because limit always cut results first and we can not get what we want.

Example:
we having export job1 and job2 with JobId1 > JobId2. We want to get job with JobId1
show export from db order by JobId desc limit 1;
We do limit 1 first, so we would probably get Job2 because JobId assigned from small to large

Solve:
We can not cut results first if we have order by clause. And cut result set after sorting

fe/fe-core/src/main/java/org/apache/doris/load/ExportMgr.java

@@ -307,7 +307,7 @@ public class ExportMgr extends MasterDaemon {
                     exportJobInfos.add(composeExportJobInfo(job));
                 }
 
-                if (++counter >= resultNum) {
+                if (++counter >= resultNum && orderByPairs == null) {
                     break;
                 }
             }
@@ -327,8 +327,12 @@ public class ExportMgr extends MasterDaemon {
         Collections.sort(exportJobInfos, comparator);
 
         List<List<String>> results = Lists.newArrayList();
+        int counter = 0;
         for (List<Comparable> list : exportJobInfos) {
             results.add(list.stream().map(e -> e.toString()).collect(Collectors.toList()));
+            if (++counter >= resultNum) {
+                break;
+            }
         }
 
         return results;

regression-test/suites/export_p0/test_export_basic.groovy

@@ -386,4 +386,47 @@ suite("test_export_basic", "p0") {
         try_sql("DROP TABLE IF EXISTS ${table_load_name}")
         delete_files.call("${outFilePath}")
     }
+
+    // 5. test order by and limit clause
+    uuid1 = UUID.randomUUID().toString()
+    outFilePath = """${outfile_path_prefix}_${uuid1}"""
+    label1 = "label_${uuid1}"
+    uuid2 = UUID.randomUUID().toString()
+    label2 = "label_${uuid2}"
+    try {
+        // check export path
+        check_path_exists.call("${outFilePath}")
+
+        // exec export
+        sql """
+            EXPORT TABLE ${table_export_name} PARTITION (less_than_20)
+            TO "file://${outFilePath}/"
+            PROPERTIES(
+                "label" = "${label1}",
+                "format" = "csv",
+                "column_separator"=","
+            );
+        """
+        sql """
+            EXPORT TABLE ${table_export_name} PARTITION (between_20_70)
+            TO "file://${outFilePath}/"
+            PROPERTIES(
+                "label" = "${label2}",
+                "format" = "csv",
+                "column_separator"=","
+            );
+        """
+        waiting_export.call(label1)
+        waiting_export.call(label2)
+
+        // check file amounts
+        check_file_amounts.call("${outFilePath}", 2)
+
+        // check show export correctness
+        def res = sql """ show export where STATE = "FINISHED" order by JobId desc limit 2"""
+        assertTrue(res[0][0] > res[1][0])
+
+    } finally {
+        delete_files.call("${outFilePath}")
+    }
 }