SERVER-12460 faster count for simple queries

1 files changed, 192 insertions, 0 deletions

diff --git a/src/mongo/db/exec/count.cpp b/src/mongo/db/exec/count.cpp
new file mode 100644
index 00000000000..4f3600fbb76
--- /dev/null
+++ b/src/mongo/db/exec/count.cpp
@@ -0,0 +1,192 @@
+/**
+ *    Copyright (C) 2014 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/db/exec/count.h"
+
+#include "mongo/db/index/index_cursor.h"
+#include "mongo/db/index/index_descriptor.h"
+
+namespace mongo {
+
+    Count::Count(const CountParams& params, WorkingSet* workingSet)
+        : _workingSet(workingSet),
+          _descriptor(params.descriptor),
+          _iam(params.descriptor->getIndexCatalog()->getIndex(params.descriptor)),
+          _btreeCursor(NULL),
+          _params(params),
+          _hitEnd(false),
+          _shouldDedup(params.descriptor->isMultikey()) { }
+
+    void Count::initIndexCursor() {
+        CursorOptions cursorOptions;
+        cursorOptions.direction = CursorOptions::INCREASING;
+
+        IndexCursor *cursor;
+        Status s = _iam->newCursor(&cursor);
+        verify(s.isOK());
+        verify(cursor);
+
+        // Is this assumption always valid?  See SERVER-12397
+        _btreeCursor.reset(static_cast<BtreeIndexCursor*>(cursor));
+        _btreeCursor->setOptions(cursorOptions);
+
+        // _btreeCursor points at our start position.  We move it forward until it hits a cursor
+        // that points at the end.
+        _btreeCursor->seek(_params.startKey, !_params.startKeyInclusive);
+
+        // Create the cursor that points at our end position.
+        IndexCursor* endCursor;
+        verify(_iam->newCursor(&endCursor).isOK());
+        verify(endCursor);
+        // Is this assumption always valid?  See SERVER-12397
+        _endCursor.reset(static_cast<BtreeIndexCursor*>(endCursor));
+        _endCursor->setOptions(cursorOptions);
+        // If the end key is inclusive we want to point *past* it since that's the end.
+        _endCursor->seek(_params.endKey, _params.endKeyInclusive);
+
+        // See if we've hit the end already.
+        checkEnd();
+    }
+
+    void Count::checkEnd() {
+        if (isEOF()) { return; }
+
+        // See if we're already past our end key.
+        int cmp = _btreeCursor->getKey().woCompare(_params.endKey, _descriptor->keyPattern(), false);
+        if (cmp > 0 || (cmp == 0 && !_params.endKeyInclusive)) {
+            _hitEnd = true;
+            return;
+        }
+
+        // If not, we're only done when we hit the end cursor.
+        _hitEnd = _btreeCursor->pointsAt(*_endCursor.get());
+    }
+
+    PlanStage::StageState Count::work(WorkingSetID* out) {
+        ++_commonStats.works;
+        if (NULL == _btreeCursor.get()) {
+            // First call to work().  Perform cursor init.
+            initIndexCursor();
+            checkEnd();
+            ++_commonStats.needTime;
+            return PlanStage::NEED_TIME;
+        }
+
+        if (isEOF()) { return PlanStage::IS_EOF; }
+
+        DiskLoc loc = _btreeCursor->getValue();
+        _btreeCursor->next();
+        checkEnd();
+
+        if (_shouldDedup) {
+            if (_returned.end() != _returned.find(loc)) {
+                ++_commonStats.needTime;
+                return PlanStage::NEED_TIME;
+            }
+            else {
+                _returned.insert(loc);
+            }
+        }
+
+        ++_commonStats.advanced;
+        *out = WorkingSet::INVALID_ID;
+        return PlanStage::ADVANCED;
+    }
+
+    bool Count::isEOF() {
+        if (NULL == _btreeCursor.get()) {
+            // Have to call work() at least once.
+            return false;
+        }
+
+        return _hitEnd || _btreeCursor->isEOF();
+    }
+
+    void Count::prepareToYield() {
+        ++_commonStats.yields;
+        if (isEOF() || (NULL == _btreeCursor.get())) { return; }
+
+        verify(!_btreeCursor->isEOF());
+        _btreeCursor->savePosition();
+        if (!_endCursor->isEOF()) {
+            _endCursor->savePosition();
+        }
+    }
+
+    void Count::recoverFromYield() {
+        ++_commonStats.unyields;
+
+        if (isEOF() || (NULL == _btreeCursor.get())) { return; }
+
+        if (!_btreeCursor->restorePosition().isOK()) {
+            _hitEnd = true;
+        }
+
+        if (!_endCursor->isEOF()) {
+            if (!_endCursor->restorePosition().isOK()) {
+                _hitEnd = true;
+            }
+        }
+        else {
+            // If we were EOF when we yielded we don't always want to have _btreeCursor run until
+            // EOF.  New documents may have been inserted after our endKey and our end marker
+            // may be before them.
+            //
+            // As an example, say we're counting from 5 to 10 and the index only has keys
+            // for 6, 7, 8, and 9.  btreeCursor will point at a 6 key at the start and the
+            // endCursor will be EOF.  If we insert documents with keys 11 during a yield we
+            // need to relocate the endCursor to point at them as the "end key" of our count.
+            _endCursor->seek(_params.endKey, _params.endKeyInclusive);
+        }
+
+        _shouldDedup = _descriptor->isMultikey();
+        checkEnd();
+    }
+
+    void Count::invalidate(const DiskLoc& dl, InvalidationType type) {
+        // 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) {
+            return;
+        }
+
+        // If we see this DiskLoc again, it may not be the same document it was before, so we want
+        // to return it if we see it again.
+        unordered_set<DiskLoc, DiskLoc::Hasher>::iterator it = _returned.find(dl);
+        if (it != _returned.end()) {
+            _returned.erase(it);
+        }
+    }
+
+    PlanStageStats* Count::getStats() {
+        _commonStats.isEOF = isEOF();
+        auto_ptr<PlanStageStats> ret(new PlanStageStats(_commonStats, STAGE_COUNT));
+        return ret.release();
+    }
+
+}  // namespace mongo