FE Plan case
Most of the test cases in this directory are to check the string of plan. Due to the orthogonality of
various syntax, it's difficult to define a clear boundary only by considering the syntax of SQL. Here
are some requirements for test cases.

##Rules
Priority:
1. According to the classification of test purpose, e.g. test right join + right table with limit.
   The test purpose is strongly related to limit and can be added it to LimitTest
2. According to the classification of function modules, e.g. the new array type, the low cardinality, can
   be add as a new test set separately

Pay attention:
1. Forbidden to add overly complex test cases. unit cases should use simple SQL that can cover the
   problem rather than complex SQL, which will increase the difficulty of repairing unit cases
2. Use meaningful test names instead of simple names without meaning

## Classification
Current classification, including & but not limited:
* Aggregate
* ArrayType
* CTE
* DecimalType
* Digest
* ExternalTable
* GroupingSet
* Join
* JsonType
* Limit
* LiteralJoin
* LowCardinality
* MetricType
* OrderBy
* Scan
* Set(UNION/INTERSECT/EXCEPT)
* Subquery
* UDF
* View
* Window