This option allows mongos to send partial query results if a shard is unreachable. This is only applicable when running a sharded MongoDB cluster and connecting to a mongos.
If a shard goes down and a query needs to be sent to that shard, mongos will return the results (if any) from shards it already contacted, then an error message that it could not reach the shard (a MongoCursorException in PHP). If you would like to get whatever results mongos can provide and no exception, you can use this method. Note that this means that you won't have an indication that a shard is down in your query response.
This has no effect on the query if all shards are reachable. This flag was implemented in MongoDB version 1.7.5, so will only work with that version and higher.
If receiving partial results is okay.
Returns this cursor.
Throws MongoCursorException if this cursor has started iterating.
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