SERVER-14097 SERVER-14098 execution-level explain for .find() and .count()

The explain implementation for .find() and .count() is feature complete. To use
the .find() implementation, set the enableNewExplain setParameter to true.

Count operations are explained through the new explain command, e.g.
db.runCommand({explain: {count: "coll", query: {foo: "bar"}}}).

1 files changed, 35 insertions, 5 deletions

diff --git a/src/mongo/db/exec/count.cpp b/src/mongo/db/exec/count.cpp
index 56e8607a611..eeb860e9e92 100644
--- a/src/mongo/db/exec/count.cpp
+++ b/src/mongo/db/exec/count.cpp
@@ -44,7 +44,10 @@ namespace mongo {
           _params(params),
           _hitEnd(false),
           _shouldDedup(params.descriptor->isMultikey()),
-          _commonStats(kStageType) { }
+          _commonStats(kStageType) {
+        _specificStats.keyPattern = _params.descriptor->keyPattern();
+        _specificStats.isMultiKey = _params.descriptor->isMultikey();
+    }
 
     void Count::initIndexCursor() {
         CursorOptions cursorOptions;
@@ -62,6 +65,8 @@ namespace mongo {
         // that points at the end.
         _btreeCursor->seek(_params.startKey, !_params.startKeyInclusive);
 
+        ++_specificStats.keysExamined;
+
         // Create the cursor that points at our end position.
         IndexCursor* endCursor;
         verify(_iam->newCursor(cursorOptions, &endCursor).isOK());
@@ -73,6 +78,8 @@ namespace mongo {
         // If the end key is inclusive we want to point *past* it since that's the end.
         _endCursor->seek(_params.endKey, _params.endKeyInclusive);
 
+        ++_specificStats.keysExamined;
+
         // See if we've hit the end already.
         checkEnd();
     }
@@ -91,10 +98,16 @@ namespace mongo {
     }
 
     PlanStage::StageState Count::work(WorkingSetID* out) {
+        ++_commonStats.works;
+
+        // Adds the amount of time taken by work() to executionTimeMillis.
+        ScopedTimer timer(&_commonStats.executionTimeMillis);
+
         if (NULL == _btreeCursor.get()) {
             // First call to work().  Perform cursor init.
             initIndexCursor();
             checkEnd();
+            ++_commonStats.needTime;
             return PlanStage::NEED_TIME;
         }
 
@@ -104,8 +117,11 @@ namespace mongo {
         _btreeCursor->next();
         checkEnd();
 
+        ++_specificStats.keysExamined;
+
         if (_shouldDedup) {
             if (_returned.end() != _returned.find(loc)) {
+                ++_commonStats.needTime;
                 return PlanStage::NEED_TIME;
             }
             else {
@@ -114,6 +130,7 @@ namespace mongo {
         }
 
         *out = WorkingSet::INVALID_ID;
+        ++_commonStats.advanced;
         return PlanStage::ADVANCED;
     }
 
@@ -127,6 +144,7 @@ namespace mongo {
     }
 
     void Count::prepareToYield() {
+        ++_commonStats.yields;
         if (_hitEnd || (NULL == _btreeCursor.get())) { return; }
 
         _btreeCursor->savePosition();
@@ -134,6 +152,7 @@ namespace mongo {
     }
 
     void Count::recoverFromYield() {
+        ++_commonStats.unyields;
         if (_hitEnd || (NULL == _btreeCursor.get())) { return; }
 
         if (!_btreeCursor->restorePosition().isOK()) {
@@ -178,6 +197,8 @@ namespace mongo {
     }
 
     void Count::invalidate(const DiskLoc& dl, InvalidationType type) {
+        ++_commonStats.invalidates;
+
         // The only state we're responsible for holding is what DiskLocs to drop.  If a document
         // mutates the underlying index cursor will deal with it.
         if (INVALIDATION_MUTATION == type) {
@@ -192,11 +213,20 @@ namespace mongo {
         }
     }
 
+    vector<PlanStage*> Count::getChildren() const {
+        vector<PlanStage*> empty;
+        return empty;
+    }
+
     PlanStageStats* Count::getStats() {
-        // We don't collect stats since this stage is only used by the count command.
-        // If count ever collects stats we must implement this.
-        invariant(0);
-        return NULL;
+        _commonStats.isEOF = isEOF();
+        auto_ptr<PlanStageStats> ret(new PlanStageStats(_commonStats, STAGE_COUNT));
+
+        CountStats* countStats = new CountStats(_specificStats);
+        countStats->keyPattern = _specificStats.keyPattern.getOwned();
+        ret->specific.reset(countStats);
+
+        return ret.release();
     }
 
 }  // namespace mongo