SERVER-5858 temporary fix so hashed indexes work with $in queries

jstests/hashindex1.js

@@ -46,14 +46,10 @@ assert.neq( t.find({c : 1}).explain().cursor ,
 		cursorname ,
 		"using irrelevant hashed cursor");
 
-/* This test should work but doesn't yet because of the way queries are
- * simplified before being passed to 'suitability'.  When this issue is
- * fixed this test can be added.
- *
-  assert.eq( t.find({a : {$in : [1,2]}}).explain()["cursor"] ,
-		cursorname ,
-		"not using hashed cursor");
-*/
+assert.eq( t.find({a : {$in : [1,2]}}).explain()["cursor"] ,
+           "BtreeCursor a_hashed multi" ,
+           "not using hashed cursor");
+
 
 //test creation of index based on hash of _id index
 var goodspec2 = {'_id' : "hashed"};

src/mongo/db/queryoptimizer.cpp

@@ -1611,7 +1611,13 @@ namespace mongo {
             // No matches are possible in the index so the index may be useful.
             return true;   
         }
-        return d->idx( idxNo ).getSpec().suitability( frsp.simplifiedQueryForIndex( d, idxNo, keyPattern ), order ) != USELESS;
+        // Hashed index types can't use simplified query bounds, since they could turn equalities
+        // into ranges, e.g.{$in : [1,2] } into {$gte : 1 , $lte : 2}
+        // TODO: refactor suitability to take a FieldRangeSetPair, and get rid of this special case
+        // See SERVER-5858.
+        BSONObj query = ( d->idx( idxNo ).getSpec().getTypeName() == "hashed" ) ?
+                        frsp.originalQuery() : frsp.simplifiedQueryForIndex( d, idxNo, keyPattern );
+        return d->idx( idxNo ).getSpec().suitability( query, order ) != USELESS;
     }
 
     void QueryUtilIndexed::clearIndexesForPatterns( const FieldRangeSetPair &frsp, const BSONObj &order ) {