[TS-207]<fix>(query): fixed compare_a function compare with length

src/query/src/qExtbuffer.c

@@ -388,30 +388,37 @@ int32_t columnValueAscendingComparator(char *f1, char *f2, int32_t type, int32_t
       int32_t len1 = varDataLen(f1);
       int32_t len2 = varDataLen(f2);
 
-      if (len1 != len2) {
-        return len1 > len2? 1:-1;
-      } else {
+      // compare zero length
+      if (len1 == 0 && len2 == 0)
+        return 0;
+      if(len1 == 0)
+        return -1;
+      if(len2 == 0)
+        return 1;
+      // compare context
       int32_t ret = strncmp(varDataVal(f1), varDataVal(f2), len1);
       if (ret == 0) {
         return 0;
       }
       return (ret < 0) ? -1 : 1;
-      }
-
     };
     case TSDB_DATA_TYPE_NCHAR: { // todo handle the var string compare
       int32_t len1 = varDataLen(f1);
       int32_t len2 = varDataLen(f2);
 
-      if (len1 != len2) {
-        return len1 > len2 ? 1 : -1;
-      } else {
+      // compare zero length
+      if (len1 == 0 && len2 == 0)
+        return 0;
+      if(len1 == 0)
+        return -1;
+      if(len2 == 0)
+        return 1;
+      // compare context  
       int32_t ret = tasoUcs4Compare(varDataVal(f1), varDataVal(f2), len1);
       if (ret == 0) {
         return 0;
       }
       return (ret < 0) ? -1 : 1;
-      }
     };
     case TSDB_DATA_TYPE_UTINYINT:  DEFAULT_COMP(GET_UINT8_VAL(f1), GET_UINT8_VAL(f2));
     case TSDB_DATA_TYPE_USMALLINT: DEFAULT_COMP(GET_UINT16_VAL(f1), GET_UINT16_VAL(f2));