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// count.cpp
/**
* Copyright (C) 2008 10gen 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/>.
*/
#include "mongo/db/ops/count.h"
#include "mongo/client/dbclientinterface.h"
#include "mongo/db/client.h"
#include "mongo/db/clientcursor.h"
#include "mongo/db/query_optimizer.h"
#include "mongo/db/queryutil.h"
#include "mongo/util/elapsed_tracker.h"
namespace mongo {
namespace {
/**
* Specialized Cursor creation rules that the count operator provides to the query
* processing system. These rules limit the performance overhead when counting index keys
* matching simple predicates. See SERVER-1752.
*/
class CountPlanPolicies : public QueryPlanSelectionPolicy {
virtual string name() const { return "CountPlanPolicies"; }
virtual bool requestMatcher() const {
// Avoid using a Matcher when a Cursor will exactly match a query.
return false;
}
virtual bool requestIntervalCursor() const {
// Request use of an IntervalBtreeCursor when the index bounds represent a single
// btree interval. This Cursor implementation is optimized for performing counts
// between two endpoints.
return true;
}
} _countPlanPolicies;
}
long long runCount( const char *ns, const BSONObj &cmd, string &err, int &errCode ) {
Client::Context cx(ns);
NamespaceDetails *d = nsdetails( ns );
if ( !d ) {
err = "ns missing";
return -1;
}
BSONObj query = cmd.getObjectField("query");
// count of all objects
if ( query.isEmpty() ) {
return applySkipLimit( d->numRecords(), cmd );
}
long long count = 0;
long long skip = cmd["skip"].numberLong();
long long limit = cmd["limit"].numberLong();
if( limit < 0 ){
limit = -limit;
}
shared_ptr<Cursor> cursor = getOptimizedCursor( ns, query, BSONObj(), _countPlanPolicies );
ClientCursorHolder ccPointer;
ElapsedTracker timeToStartYielding( 256, 20 );
try {
while( cursor->ok() ) {
if ( !ccPointer ) {
if ( timeToStartYielding.intervalHasElapsed() ) {
// Lazily construct a ClientCursor, avoiding a performance regression when scanning a very
// small number of documents.
ccPointer.reset( new ClientCursor( QueryOption_NoCursorTimeout, cursor, ns ) );
}
}
else if ( !ccPointer->yieldSometimes( ClientCursor::MaybeCovered ) ||
!cursor->ok() ) {
break;
}
if ( cursor->currentMatches() && !cursor->getsetdup( cursor->currLoc() ) ) {
if ( skip > 0 ) {
--skip;
}
else {
++count;
if ( limit > 0 && count >= limit ) {
break;
}
}
}
cursor->advance();
}
ccPointer.reset();
return count;
}
catch ( const DBException &e ) {
err = e.toString();
errCode = e.getCode();
}
catch ( const std::exception &e ) {
err = e.what();
errCode = 0;
}
// Historically we have returned zero in many count assertion cases - see SERVER-2291.
log() << "Count with ns: " << ns << " and query: " << query
<< " failed with exception: " << err << " code: " << errCode
<< endl;
return -2;
}
} // namespace mongo
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