without default database explain failed even referring database at table quote

[irene-coming]

dble version:5.6.29-dble-9.9.9.9-08136d5-20181106070356
schema.xml

<schema name="testdb">
<table name="sharding_two_node" dataNode="dn1,dn2" rule="hash-two"/>
<table name="sharding_two_node2" dataNode="dn1,dn2" rule="hash-two"/>
</schema>

steps:
step1. login mysql without assign default db(don't use -Ddbname)
step2. execute D.1(or D.2):

mysql> explain select * from testdb.sharding_two_node a join testdb2.sharding_two_node2 b on a.id = b.id where a.id =1 and b.id=1;
ERROR 1046 (HY000): No database selected
mysql> show databases;
+----------+
| DATABASE |
+----------+
| testdb     |
| testdb2   |
+----------+
2 rows in set (0.00 sec)

expect:
step2 "explain ... " get success like:

+-----------+----------+-----------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE     | SQL/REF                                                                                             |
+-----------+----------+-----------------------------------------------------------------------------------------------------+
| dn1       | BASE SQL | select * from sharding_two_node a join sharding_two_node2 b on a.id = b.id where a.id =1 and b.id=1 |
+-----------+----------+-----------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

supplements:
issue context is same as (#752)

[FlyingMao]

verified version: 5.6.29-dble-9.9.9.9-475ffa1-20181121114114