[FYI] JSON query execution is available

[Jihoon Son]

Hi folks,

Tajo now supports the query execution of JSON form.
Here is an example of JSON representation of a SQL query.

SQL> select count(*) from region

JSON>
{
  "IsDistinct": false,
  "Projections": [
    {
      "Expr": {
        "IsDistinct": false,
        "Signature": "count",
        "FuncParams": [],
        "OpType": "CountRowsFunction"
      },
      "OpType": "Target"
    }
  ],
  "Expr": {
    "Relations": [
      {
        "TableName": "region",
        "OpType": "Relation"
      }
    ],
    "OpType": "RelationList"
  },
  "OpType": "Projection"
}

More detailed information of JSON representation can be found at
https://cwiki.apache.org/confluence/display/TAJO/JSON+expression+of+relational+operators
.

To execute JSON queries, executeQueryWithJson() is added to TajoClient.
In addition, JSON queries can be currently executed via TajoCli. We plan to
provide a simple JSON cli, soon (TAJO-826).

I expect that this feature will be useful for various OLAP applications
which use TAJO as their query engine.

Thanks,
Jihoon