MongoDB Documentation, Release 3.0.4 • The reIndex rebuilds the index. • You add or drop an index. • The mongod process restarts. Changed in version 2.6: explain() operations no longer read from or write to the query planner cache. New in version 2.6. MongoDB provides http://docs.mongodb.org/manual/reference/method/js-plan-cache to view and modify the cached query plans. New in version 2.6. Index filters determine which indexes the optimizer evaluates for a query shape . A query shape consists of a combi- nation of query, sort, and projection specifications. If an index filter exists for a given query shape, the optimizer only considers those indexes specified in the filter. When an index filter exists for the query shape, MongoDB ignores the hint() . To see whether MongoDB applied an index filter for a query shape, check the indexFilterSet field of either the db.collection.explain() or the cursor.explain() method. Index filters only affects which indexes the optimizer evaluates; the optimizer may still select the collection scan as the winning plan for a given query shape. Index filters exist for the duration of the server process and do not persist after shutdown. MongoDB also provides a command to manually remove filters. Because index filters overrides the expected behavior of the optimizer as well as the hint() method, use index filters sparingly. See planCacheListFilters , planCacheClearFilters , and planCacheSetFilter . Sharded clusters allow you to partition a data set among a cluster of mongod instances in a way that is nearly trans- parent to the application. For an overview of sharded clusters, see the Sharding (page 661) section of this manual. For a sharded cluster, applications issue operations to one of the mongos instances associated with the cluster. Read operations on sharded clusters are most efficient when directed to a specific shard. Queries to sharded collections should include the collection’s shard key (page 674). When a query includes a shard key, the mongos can use cluster metadata from the config database (page 670) to route the queries to shards. If a query does not include the shard key, the mongos must direct the query to all shards in the cluster. These scatter gather queries can be inefficient. On larger clusters, scatter gather queries are unfeasible for routine operations. For more information on read operations in sharded clusters, see the Sharded Cluster Query Routing (page 678) and Shard Keys (page 674) sections. 69 3.2. MongoDB CRUD Concepts Distributed Queries Read Operations to Sharded Clusters Index Filters Cached Query Plan Interface