db.coll.find({}, ..., {showRecordId: true}).sort({_id: 1}).forEach(print(_id + ": " recordId));
1: 1
2: 605003
3: 1277000
4: 1805464
5: 2
2: 605004
3: 1277001
...
mongodump
mongorestore
db.coll.find({}, ..., {showRecordId: true}).sort({_id: 1}).forEach(print(_id + ": " recordId));
1: 1
2: 621037
3: 1284322
4: 1791763
5: 2
2: 621038
3: 1284324
...
//(same story at high _id values, not just from 1 ~)

void fillOutPlannerParams(OperationContext* txn,
                          Collection* collection,
                          CanonicalQuery* canonicalQuery,
                          QueryPlannerParams* plannerParams) {
...
...

    // MMAPv1 storage engine should have snapshot() perform an index scan on _id rather than a
    // collection scan since a collection scan on the MMAP storage engine can return duplicates
    // or miss documents.
    if (isMMAPV1()) {
        plannerParams->options |= QueryPlannerParams::SNAPSHOT_USE_ID;
    }
}

Status QueryPlanner::plan(const CanonicalQuery& query,
                          const QueryPlannerParams& params,
                          std::vector<QuerySolution*>* out) {
    ...
    ...

    // If snapshot is set, default to collscanning. If the query param SNAPSHOT_USE_ID is set,
    // snapshot is a form of a hint, so try to use _id index to make a real plan. If that fails,
    // just scan the _id index.
    //
    // Don't do this if the query is a geonear or text as as text search queries must be answered
    // using full text indices and geoNear queries must be answered using geospatial indices.
    if (query.getParsed().isSnapshot() &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR) &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::TEXT)) {
        const bool useIXScan = params.options & QueryPlannerParams::SNAPSHOT_USE_ID;

        if (!useIXScan) {
            QuerySolution* soln = buildCollscanSoln(query, isTailable, params);
            if (soln) {
                out->push_back(soln);
            }
            return Status::OK();
        } else {
            // Find the ID index in indexKeyPatterns. It's our hint.
            for (size_t i = 0; i < params.indices.size(); ++i) {
                if (isIdIndex(params.indices[i].keyPattern)) {
                    hintIndex = params.indices[i].keyPattern;
                    break;
                }
            }
        }
    }

    ...
}