Query explanation

When troubleshooting kylin query, we could use "Insight" tab to issue query, then checking the kylin.log. This definitely the right way, but make it only available to kylin admin who can access the kylin log.

Since most query system provide query explanation(same as calcite), Kylin might also output query explanation in response. Say when issue query with explain=true , the query response would return structured explanation, like how many htable touched, how many records read from each htable and the time consumption; and cache hit status

Both kylin owner and user should benefit from such feature.