SERVER-20536 Ignore comment field when planning counts.

1 files changed, 31 insertions, 5 deletions

diff --git a/src/mongo/db/exec/count.h b/src/mongo/db/exec/count.h
index e083eb59aff..56cdbe6614c 100644
--- a/src/mongo/db/exec/count.h
+++ b/src/mongo/db/exec/count.h
@@ -34,6 +34,30 @@
 
 namespace mongo {
 
+struct CountStageParams {
+    CountStageParams(const CountRequest& request, bool useRecordStoreCount)
+        : nss(request.getNs()),
+          limit(request.getLimit()),
+          skip(request.getSkip()),
+          useRecordStoreCount(useRecordStoreCount) {}
+
+    // Namespace to operate on (e.g. "foo.bar").
+    NamespaceString nss;
+
+    // An integer limiting the number of documents to count. 0 means no limit.
+    long long limit;
+
+    // An integer indicating to not include the first n documents in the count. 0 means no skip.
+    long long skip;
+
+    // True if this count stage should just ask the record store for a count instead of computing
+    // one itself.
+    //
+    // Note: This strategy can lead to inaccurate counts on certain storage engines (including
+    // WiredTiger).
+    bool useRecordStoreCount;
+};
+
 /**
  * Stage used by the count command. This stage sits at the root of a plan tree
  * and counts the number of results returned by its child stage.
@@ -49,7 +73,7 @@ class CountStage final : public PlanStage {
 public:
     CountStage(OperationContext* txn,
                Collection* collection,
-               const CountRequest& request,
+               CountStageParams params,
                WorkingSet* ws,
                PlanStage* child);
 
@@ -68,15 +92,17 @@ public:
 
 private:
     /**
-     * Computes the count in the case of an empty query, applying the skip and
-     * limit if necessary. The result is stored in '_specificStats'.
+     * Asks the record store for the count, applying the skip and limit if necessary. The result is
+     * stored in '_specificStats'.
+     *
+     * This is only valid if the query and hint are both empty.
      */
-    void trivialCount();
+    void recordStoreCount();
 
     // The collection over which we are counting.
     Collection* _collection;
 
-    CountRequest _request;
+    CountStageParams _params;
 
     // The number of documents that we still need to skip.
     long long _leftToSkip;