SERVER-40755 Expose statistics which indicate how many collection scans have executed

1 files changed, 20 insertions, 1 deletions

diff --git a/src/mongo/db/collection_index_usage_tracker.h b/src/mongo/db/collection_index_usage_tracker.h
index 62c3b610f41..b70d7887723 100644
--- a/src/mongo/db/collection_index_usage_tracker.h
+++ b/src/mongo/db/collection_index_usage_tracker.h
@@ -44,6 +44,9 @@ class ClockSource;
  * considered "used" when it appears as part of a winning plan for an operation that uses the
  * query system.
  *
+ * It also tracks non-usage of indexes. I.e. it collects information about collection scans that
+ * occur on a collection.
+ *
  * Indexes must be registered and deregistered on creation/destruction.
  */
 class CollectionIndexUsageTracker {
@@ -51,6 +54,11 @@ class CollectionIndexUsageTracker {
     CollectionIndexUsageTracker& operator=(const CollectionIndexUsageTracker&) = delete;
 
 public:
+    struct CollectionScanStats {
+        unsigned long long collectionScans{0};
+        unsigned long long collectionScansNonTailable{0};
+    };
+
     struct IndexUsageStats {
         IndexUsageStats() = default;
         explicit IndexUsageStats(Date_t now, const BSONObj& key)
@@ -111,6 +119,15 @@ public:
      */
     StringMap<CollectionIndexUsageTracker::IndexUsageStats> getUsageStats() const;
 
+    /**
+     * Get the current state of the usage of collection scans. This struct will only include
+     * information about the collection scans that have occured at the time of calling.
+     */
+    CollectionScanStats getCollectionScanStats() const;
+
+    void recordCollectionScans(unsigned long long collectionScans);
+    void recordCollectionScansNonTailable(unsigned long long collectionScansNonTailable);
+
 private:
     // Map from index name to usage statistics.
     StringMap<CollectionIndexUsageTracker::IndexUsageStats> _indexUsageMap;
@@ -118,8 +135,10 @@ private:
     // Clock source. Used when the 'trackerStartTime' time for an IndexUsageStats object needs to
     // be set.
     ClockSource* _clockSource;
+
+    AtomicWord<unsigned long long> _collectionScans{0};
+    AtomicWord<unsigned long long> _collectionScansNonTailable{0};
 };
 
 typedef StringMap<CollectionIndexUsageTracker::IndexUsageStats> CollectionIndexUsageMap;
-
 }  // namespace mongo