Edit: changed these comments:

Most important is to change the WHERE to not use functions on the table column.

You should also very strongly analyze clustering the table by INSERT_DETECT_TS (add Incident_Id to the clus key if you want), if it's not already.

SELECT DISTINCT Incident_id, policy_name

FROM Reporting_DailyDlpDetail

WHERE INSERT_DETECT_TS >= '20150602' AND INSERT_DETECT_TS < '20150610'

If you only need the total, you can add an outer query:

SELECT COUNT(*) AS Distinct_Total

FROM (

SELECT DISTINCT Incident_id, policy_name

FROM Reporting_DailyDlpDetail

WHERE INSERT_DETECT_TS >= '20150602' AND INSERT_DETECT_TS < '20150610'

) AS derived