SERVER-24242 Stop applying the MatchExpression to OplogReplay queries after first match

3 files changed, 19 insertions, 7 deletions

diff --git a/src/mongo/db/exec/collection_scan.cpp b/src/mongo/db/exec/collection_scan.cpp
index b130d128023..1e1e88ead01 100644
--- a/src/mongo/db/exec/collection_scan.cpp
+++ b/src/mongo/db/exec/collection_scan.cpp
@@ -171,6 +171,9 @@ PlanStage::StageState CollectionScan::returnIfMatches(WorkingSetMember* member,
     ++_specificStats.docsTested;
 
     if (Filter::passes(member, _filter)) {
+        if (_params.stopApplyingFilterAfterFirstMatch) {
+            _filter = nullptr;
+        }
         *out = memberID;
         return PlanStage::ADVANCED;
     } else {
diff --git a/src/mongo/db/exec/collection_scan_common.h b/src/mongo/db/exec/collection_scan_common.h
index f5766e0a0d6..712e56f1eac 100644
--- a/src/mongo/db/exec/collection_scan_common.h
+++ b/src/mongo/db/exec/collection_scan_common.h
@@ -40,24 +40,25 @@ struct CollectionScanParams {
         BACKWARD = -1,
     };
 
-    CollectionScanParams()
-        : collection(NULL), start(RecordId()), direction(FORWARD), tailable(false), maxScan(0) {}
-
     // What collection?
     // not owned
-    const Collection* collection;
+    const Collection* collection = nullptr;
 
     // isNull by default.  If you specify any value for this, you're responsible for the RecordId
     // not being invalidated before the first call to work(...).
     RecordId start;
 
-    Direction direction;
+    Direction direction = FORWARD;
 
     // Do we want the scan to be 'tailable'?  Only meaningful if the collection is capped.
-    bool tailable;
+    bool tailable = false;
+
+    // Once the first matching document is found, assume that all documents after it must match.
+    // This is useful for oplog queries where we know we will see records ordered by the ts field.
+    bool stopApplyingFilterAfterFirstMatch = false;
 
     // If non-zero, how many documents will we look at?
-    size_t maxScan;
+    size_t maxScan = 0;
 };
 
 }  // namespace mongo
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index 4258c1f9ab9..b4e9ca6de89 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -597,6 +597,14 @@ StatusWith<unique_ptr<PlanExecutor>> getOplogStartHack(OperationContext* txn,
     params.direction = CollectionScanParams::FORWARD;
     params.tailable = cq->getQueryRequest().isTailable();
 
+    // If the query is just tsExpr, we know that every document in the collection after the first
+    // matching one must also match. To avoid wasting time running the match expression on every
+    // document to be returned, we tell the CollectionScan stage to stop applying the filter once it
+    // finds the first match.
+    if (cq->root() == tsExpr) {
+        params.stopApplyingFilterAfterFirstMatch = true;
+    }
+
     unique_ptr<WorkingSet> ws = make_unique<WorkingSet>();
     unique_ptr<CollectionScan> cs = make_unique<CollectionScan>(txn, params, ws.get(), cq->root());
     // Takes ownership of 'ws', 'cs', and 'cq'.