Acessing PostgresSQL DB from TW GetDataTableEntries

We have installed the Postgres version of TW 7.1. We have also created a new Postgres based persistence provider and created a new DataTable based on it. The configuration has been done properly. But when we are invoking the service GetDataTableEntries, it is throwing this exception

Unable to Invoke Service GetDataTableEntries on SIM_Postgre : com.thingworx.common.exceptions.DataAccessException: [1,018] Data store unknown error: [Error occurred while accessing the data provider.]

NOTE: We do not want to use the SQL(Query) in the service for getting the data from the database.