Execute and Display SQL Explain in query page

As a user, I would like to:
1. Execute SQL Explain Plan before execute real SQL
2. Display SQL Explain Plan with user information, such as how many rows will be scan, how many rows will be returned, and the each step's cost (if have)

---------------- Imported from GitHub ----------------
Url: https://github.com/KylinOLAP/Kylin/issues/322
Created by: lukehan
Labels: newfeature,
Milestone: v2.0 Release
Created at: Fri Dec 26 15:24:49 CST 2014
State: open