The result of `explain information functions` is incorrect

[FlyingMao]

• dble version：
  dble-9.9.9.9-9bf5140bf8483248e94077ba0b429e7d4acfb353-20200326102553
• preconditions ：
  no
• configs：

schema.xml

rule.xml

server.xml

• steps：
  step1. explain information functions in dble such as

explain SELECT ROW_COUNT();

• expect result：
  1. inner_func_merge
• real result：
  1.

mysql> explain SELECT ROW_COUNT();
+-----------+----------+--------------------+
| DATA_NODE | TYPE     | SQL/REF            |
+-----------+----------+--------------------+
| dn5       | BASE SQL | SELECT ROW_COUNT() |
+-----------+----------+--------------------+

• supplements：
  1.