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SERVER-41329 Improve skip performance in mongos when request is sent to a single shard #1326

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SERVER-41329 Improve skip performance in mongos when request is sent to a single shard #1326

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110 changes: 73 additions & 37 deletions src/mongo/s/query/cluster_find.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -88,56 +88,83 @@ static const int kPerDocumentOverheadBytesUpperBound = 10;
* suitable for forwarding to the targeted hosts.
*/
StatusWith<std::unique_ptr<QueryRequest>> transformQueryForShards(
const QueryRequest& qr, bool appendGeoNearDistanceProjection) {
const QueryRequest& qr, bool appendGeoNearDistanceProjection, bool forwardToSingleShard) {
// For multi-shards queries:
// If there is a limit, we forward the sum of the limit and the skip.
// For single-shard queries:
// If there is a limit, we forward the original limit.
boost::optional<long long> newLimit;
if (qr.getLimit()) {
long long newLimitValue;
if (mongoSignedAddOverflow64(*qr.getLimit(), qr.getSkip().value_or(0), &newLimitValue)) {
return Status(
ErrorCodes::Overflow,
str::stream()
<< "sum of limit and skip cannot be represented as a 64-bit integer, limit: "
<< *qr.getLimit()
<< ", skip: "
<< qr.getSkip().value_or(0));
if (forwardToSingleShard) {
newLimit = *qr.getLimit();
} else {
long long newLimitValue;
if (mongoSignedAddOverflow64(*qr.getLimit(), qr.getSkip().value_or(0), &newLimitValue)) {
return Status(
ErrorCodes::Overflow,
str::stream()
<< "sum of limit and skip cannot be represented as a 64-bit integer, limit: "
<< *qr.getLimit()
<< ", skip: "
<< qr.getSkip().value_or(0));
}
newLimit = newLimitValue;
}
newLimit = newLimitValue;
}

// Similarly, if nToReturn is set, we forward the sum of nToReturn and the skip.
// For multi-shards queries:
// If nToReturn is set, we forward the sum of nToReturn and the skip.
// For single-shard queries:
// If nToReturn is set, we forward the original nToReturn.
boost::optional<long long> newNToReturn;
if (qr.getNToReturn()) {
// !wantMore and ntoreturn mean the same as !wantMore and limit, so perform the conversion.
if (!qr.wantMore()) {
long long newLimitValue;
if (mongoSignedAddOverflow64(
*qr.getNToReturn(), qr.getSkip().value_or(0), &newLimitValue)) {
return Status(ErrorCodes::Overflow,
str::stream()
<< "sum of ntoreturn and skip cannot be represented as a 64-bit "
if (forwardToSingleShard) {
newLimit = *qr.getNToReturn();
} else {
long long newLimitValue;
if (mongoSignedAddOverflow64(
*qr.getNToReturn(), qr.getSkip().value_or(0), &newLimitValue)) {
return Status(ErrorCodes::Overflow,
str::stream()
<< "sum of ntoreturn and skip cannot be represented as a 64-bit "
"integer, ntoreturn: "
<< *qr.getNToReturn()
<< ", skip: "
<< qr.getSkip().value_or(0));
<< *qr.getNToReturn()
<< ", skip: "
<< qr.getSkip().value_or(0));
}
newLimit = newLimitValue;
}
newLimit = newLimitValue;
} else {
long long newNToReturnValue;
if (mongoSignedAddOverflow64(
*qr.getNToReturn(), qr.getSkip().value_or(0), &newNToReturnValue)) {
return Status(ErrorCodes::Overflow,
str::stream()
<< "sum of ntoreturn and skip cannot be represented as a 64-bit "
"integer, ntoreturn: "
<< *qr.getNToReturn()
<< ", skip: "
<< qr.getSkip().value_or(0));
if (forwardToSingleShard) {
newNToReturn = *qr.getNToReturn();
} else {
long long newNToReturnValue;
if (mongoSignedAddOverflow64(
*qr.getNToReturn(), qr.getSkip().value_or(0), &newNToReturnValue)) {
return Status(ErrorCodes::Overflow,
str::stream()
<< "sum of ntoreturn and skip cannot be represented as a 64-bit "
"integer, ntoreturn: "
<< *qr.getNToReturn()
<< ", skip: "
<< qr.getSkip().value_or(0));
}
newNToReturn = newNToReturnValue;
}
newNToReturn = newNToReturnValue;
}
}

// For single-shard queries, we forward the original skip to mongod.
// For multi-shards queries or no skip specified, we forward none skip
boost::optional<long long> newSkip;
if (qr.getSkip() && forwardToSingleShard) {
newSkip = *qr.getSkip();
} else {
newSkip = boost::none;
}

// If there is a sort other than $natural, we send a sortKey meta-projection to the remote node.
BSONObj newProjection = qr.getProj();
if (!qr.getSort().isEmpty() && !qr.getSort()["$natural"]) {
Expand All @@ -157,7 +184,7 @@ StatusWith<std::unique_ptr<QueryRequest>> transformQueryForShards(

auto newQR = stdx::make_unique<QueryRequest>(qr);
newQR->setProj(newProjection);
newQR->setSkip(boost::none);
newQR->setSkip(newSkip);
newQR->setLimit(newLimit);
newQR->setNToReturn(newNToReturn);

Expand All @@ -180,9 +207,10 @@ std::vector<std::pair<ShardId, BSONObj>> constructRequestsForShards(
const std::set<ShardId>& shardIds,
const CanonicalQuery& query,
bool appendGeoNearDistanceProjection,
boost::optional<LogicalTime> atClusterTime) {
boost::optional<LogicalTime> atClusterTime,
bool forwardToSingleShard) {
const auto qrToForward = uassertStatusOK(
transformQueryForShards(query.getQueryRequest(), appendGeoNearDistanceProjection));
transformQueryForShards(query.getQueryRequest(), appendGeoNearDistanceProjection, forwardToSingleShard));

if (atClusterTime) {
auto readConcernAtClusterTime =
Expand Down Expand Up @@ -278,11 +306,19 @@ CursorId runQueryWithoutRetrying(OperationContext* opCtx,
// Tailable cursors can't have a sort, which should have already been validated.
invariant(params.sort.isEmpty() || !query.getQueryRequest().isTailable());

bool forwardToSingleShard = false;
if (shardIds.size() == 1) {
LOG(3) << "cluster find command: " << query.toString() <<", forward to a single shard";
forwardToSingleShard = true;
// Skip nothing in mongos, as skip operation is done in mongod if needed.
params.skip = boost::none;
}

// Construct the requests that we will use to establish cursors on the targeted shards,
// attaching the shardVersion and txnNumber, if necessary.

auto requests = constructRequestsForShards(
opCtx, routingInfo, shardIds, query, appendGeoNearDistanceProjection, atClusterTime);
opCtx, routingInfo, shardIds, query, appendGeoNearDistanceProjection, atClusterTime, forwardToSingleShard);

// Establish the cursors with a consistent shardVersion across shards.

Expand Down