How can I solve the MongoDB Overflow sort stage error?

Question

I am using the MongoDB 2.6.7 in Windows 7 (64 bit) with MongoDB C# Driver 1.10.0 and I am facing the following error:

MongoDB.Driver.MongoQueryException: QueryFailure flag was getMore runner error: Overflow sort stage buffered data usage of 33554527 bytes exceeds internal limit of 33554432 bytes (response was { "$err" : "getMore runner error: Overflow sort stage buffered data usage of 33554527 bytes exceeds internal limit of 33554432 bytes", "code" : 17406 }).

The query I run in C# is:

var query = Query<User>.EQ(c => c.GroupId, groupId);

return Collection.Find(query)
                 .SetSortOrder(SortBy.Ascending(User.ID))
                 .SetSkip(page * records)
                 .SetLimit(records)
                 .ToList();

I am using an single index for GroupId property and also the ID (with _id value) property has the default index.

I see that there is a related bug in the MongoDB:

https://jira.mongodb.org/browse/SERVER-14174

How can I solve this error?

Answer 1

The server issue you've highlighted isn't really related. The error you are encountering indicates too much data (>32Mb) for an in-memory sort. Adding an appropriate index should resolve this. It looks like your User collection should have a compound index on { groupId:1, _id:1} .

Order in compound keys is definitely important. For some helpful background reading see: Optimizing MongoDB Compound Indexes .