mongoDB : Hinting index in java

DBCursor class provides hint() method, which is used to override MongoDB’s default index selection and query optimization process.

public DBCursor hint(DBObject indexKeys)

Informs the database of indexed fields of the collection in order to improve performance.

public DBCursor hint(String indexName)

Informs the database of an indexed field of the collection in order to improve performance.

> db.students.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "school.students"
        },
        {
                "v" : 1,
                "key" : {
                        "last_name" : 1
                },
                "name" : "last_name_1",
                "ns" : "school.students"
        },
        {
                "v" : 1,
                "key" : {
                        "first_name" : 1
                },
                "name" : "first_name_1",
                "ns" : "school.students"
        },
        {
                "v" : 1,
                "key" : {
                        "first_name" : 1,
                        "last_name" : 1
                },
                "name" : "first_name_1_last_name_1",
                "ns" : "school.students"
        }
]
>

import com.mongodb.BasicDBObject;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBObject;
import com.mongodb.MongoClient;

public class MongoDBEx {
  /* Step 1 : get mongoCLient */
  public static MongoClient getMongoClient() {
    MongoClient mongoClient = null;
    try {
      mongoClient = new MongoClient("localhost", 27017);
    } catch (Exception e) {
      e.printStackTrace();
    }
    return mongoClient;
  }

  public static void main(String[] args) throws Exception {
    MongoClient mongoClient = getMongoClient();

    /* Step 2: Connect to DB */
    DB db = mongoClient.getDB("school");

    /* Step 3 : Get collection */
    DBCollection collection = db.getCollection("students");

    BasicDBObject obj = new BasicDBObject().append("first_name", "first_name1");
    BasicDBObject hint = new BasicDBObject().append("first_name", 1);
    //String hint = "first_name_1";
    
    DBObject explanation = collection.find(obj).hint(hint).explain();
    
    System.out.println(explanation);
    
    /* Close client */
    mongoClient.close();
  }
}

Output

{ "cursor" : "BtreeCursor first_name_1" , "isMultiKey" : false , "n" : 1 , "nscannedObjects" : 1 , "nscanned" : 1 , "nscannedObjectsAllPlans" : 1 , "nscannedAllPlans" : 1 , "scanAndOrder" : false , "indexOnly" : false , "nYields" : 1 , "nChunkSkips" : 0 , "millis" : 73 , "indexBounds" : { "first_name" : [ [ "first_name1" , "first_name1"]]} , "allPlans" : [ { "cursor" : "BtreeCursor first_name_1" , "isMultiKey" : false , "n" : 1 , "nscannedObjects" : 1 , "nscanned" : 1 , "scanAndOrder" : false , "indexOnly" : false , "nChunkSkips" : 0 , "indexBounds" : { "first_name" : [ [ "first_name1" , "first_name1"]]}}] , "server" : "RENT-MIS-LT3016:27017" , "filterSet" : false , "stats" : { "type" : "FETCH" , "works" : 2 , "yields" : 1 , "unyields" : 1 , "invalidates" : 0 , "advanced" : 1 , "needTime" : 0 , "needFetch" : 0 , "isEOF" : 1 , "alreadyHasObj" : 0 , "forcedFetches" : 0 , "matchTested" : 0 , "children" : [ { "type" : "IXSCAN" , "works" : 2 , "yields" : 1 , "unyields" : 1 , "invalidates" : 0 , "advanced" : 1 , "needTime" : 0 , "needFetch" : 0 , "isEOF" : 1 , "keyPattern" : "{ first_name: 1.0 }" , "isMultiKey" : 0 , "boundsVerbose" : "field #0['first_name']: [\"first_name1\", \"first_name1\"]" , "yieldMovedCursor" : 0 , "dupsTested" : 0 , "dupsDropped" : 0 , "seenInvalidated" : 0 , "matchTested" : 0 , "keysExamined" : 1 , "children" : [ ]}]}}

Prevoius                                                 Next                                                 Home