use default database in subquery not support in offline

## create table in db1, then select subquery in db

db: db1
table: t1
columns : ["c1 string","c2 int","c3 bigint","c4 timestamp"]
indexs: ["index1:c1:c4"]
rows:
- ["aa",2,3,1590738989000]
- ["bb",21,31,1590738990000]
- ["cc",41,51,1590738991000]
` sql: select db.t2.c1 from (select c1, c2+1,c3+1 from db1.t1) as t2;`

The issue describes a subquery referencing a default database not being supported in offline mode. Start by examining the SQL parser and query planner for handling database qualifiers in subqueries, particularly in offline execution. Look for existing tests related to subqueries and database context to understand the expected behavior. Verify the fix by running the provided SQL example in offline mode.

Written by the indexing model from the issue text.