Drill should push down all row key filters to maprDB for queries that only have WHERE conditions on row_keys. In the following example, the query only has where clause on row_keys:
select t.mscIdentities from dfs.root.`/user/mapr/MixTable` t where t.row_key=
(select max(convert_fromutf8(i.KeyA.ENTRY_KEY)) from dfs.root.`/user/mapr/TableIMSI` i where i.row_key='460021050005636')
row_keys can return at most 1 row. So the physical planning must leverage MapRDB row_key push down to execute the sub query, with its results execute the outer query. Currently only the inner query is pushed down. The outer query requires a table scan.