// selecting a block id and data by row example in PostgreSQL:
postgres=> select ctid, * from baz;
 ctid  | id | name  
-------+----+-------
 (0,1) |  1 | kenny

// nice to have for mongodb:
> db.kg.find()
{ "_block":0001,"_id" : ObjectId("4bd6027e7d4426ce1a436e01"), "0" : { "name" : "kenny" } }