commit faae89039d43effa5ac30433c0843181e646eae1
Author: CenZheng<mingyan.zc@taobao.com>
Date:   Mon Mar 23 13:57:19 2020 +0800

    optimize getShardIdForRange for query range over[MinKey, Maxkey]

diff --git a/src/mongo/bson/bsonobj.cpp b/src/mongo/bson/bsonobj.cpp
index 5df0882cb7..970d31efb7 100644
--- a/src/mongo/bson/bsonobj.cpp
+++ b/src/mongo/bson/bsonobj.cpp
@@ -313,6 +313,19 @@ int BSONObj::getFieldNames(set<string>& fields) const {
     return n;
 }
 
+int BSONObj::areAllElemsOfType(BSONType type) const {
+    BSONObjIterator i(*this);
+    while (i.moreWithEOO()) {
+        BSONElement e = i.next();
+        if (e.eoo())
+            break;
+        if (e.type() != type) {
+            return false;
+        }
+    }
+    return true;
+}
+
 /* note: addFields always adds _id even if not specified
    returns n added not counting _id unless requested.
 */
diff --git a/src/mongo/bson/bsonobj.h b/src/mongo/bson/bsonobj.h
index 2e3665ccbe..0ae7e8ed74 100644
--- a/src/mongo/bson/bsonobj.h
+++ b/src/mongo/bson/bsonobj.h
@@ -286,6 +286,9 @@ public:
     */
     int nFields() const;
 
+    /** Are all elements of the specified type */
+    int areAllElemsOfType(BSONType type) const ;
+
     /** adds the field names to the fields set.  does NOT clear it (appends). */
     int getFieldNames(std::set<std::string>& fields) const;
 
diff --git a/src/mongo/s/chunk_manager.cpp b/src/mongo/s/chunk_manager.cpp
index e1432fe408..166e5630f9 100644
--- a/src/mongo/s/chunk_manager.cpp
+++ b/src/mongo/s/chunk_manager.cpp
@@ -189,6 +189,16 @@ void ChunkManager::getShardIdsForQuery(OperationContext* opCtx,
 void ChunkManager::getShardIdsForRange(const BSONObj& min,
                                        const BSONObj& max,
                                        std::set<ShardId>* shardIds) const {
+    // This optimization does not apply when we are reading from a snapshot
+    // because _shardVersions contains shards with chunks and is built based on the last
+    // refresh. Therefore, it is possible for _shardVersions to have fewer entries if a shard
+    // no longer owns chunks when it used to at _clusterTime.
+    if (!_clusterTime &&
+            min.areAllElemsOfType(BSONType::MinKey) &&
+            max.areAllElemsOfType(BSONType::MaxKey)) {
+        getAllShardIds(shardIds);
+        return;
+    }
     const auto bounds = _rt->overlappingRanges(min, max, true);
     for (auto it = bounds.first; it != bounds.second; ++it) {
         shardIds->insert(it->second->getShardIdAt(_clusterTime));