Can use of $regex in MongoDB Aggregation Framework be optimized? -

i'm using aggregation framework calculate aggregate values on collection. typical query/operation might include close 1 million documents indexed (the f1/ts match reduces number of documents down 1 million 5 million, set can made smaller depending on query parameters, e.g. timeframe selected).

the $match uses indexed attributes of documents, need include amounts full-text search. option i've used $regex match. unfortunately, can't anchor $regex match start of string since string(s) i'm looking anywhere. text i'm "searching" anywhere few characters in length few thousand.

just running basic comparisons, inclusion of $regex match attribute doubles time calculation takes complete.

• what options optimizing operation?
• is possible achieve other way?
• will text-search available use in aggregation operations in v2.6?

for reference:

operation without $regex

db.my_collection.aggregate(  {    $match:{      f1:objectid('417abd81...577000006'),      ts:{$gte:t1,$lte:t2}    }  },{    $project:{      ts:1,      p:1    }  },{    $group:{      _id:"$ts",      x: {        $sum:"$p"      }    }  }); 

operation $regex

db.my_collection.aggregate(  {    $match:{      f1:objectid('417abd81...577000006'),      ts:{$gte:t1,$lte:t2}    }  },  {    $match:{      c:{$regex: /somevalue/i}    }  },{    $project:{      ts:1,      p:1    }  },{    $group:{      _id:"$ts",      x: {        $sum:"$p"      }    }  });