The countDocument() command with skey scan should not lookup base table

[xiexiaoy]

> db.sbtest1.countDocuments({}, { hint: { _id: 1 } })
4000000
> db.sbtest1.countDocuments({}, { hint: { k1: 1 } })

[xiexiaoy]

TODO:

• Coverd index scan should not lookup base table.
• Filter before lookup base table.

[xiexiaoy]

For Covered Index Scan

Whether looking up a base table is decided at the plan analysis step (QueryPlannerAnalysis::analyzeDataAccess()), not the plan execution step. The IndexScan stage contains no information on whether looking up a base table.

For Filter

The call switch - _checker->checkKey(kv->key, &_seekPoint) is not a static function to be called in EloqIndexCursor. Its return value and output parameter _seekPoint are required by IndexScan::doWork.

[xiexiaoy]

I will add an _enablePrefetchRecords member to the EloqIndexCursor class, set to false by default. FetchStage::doWork() will enable it. FetchStage::doWork() will use a dynamic_cast to test if its child is a SortedDataInterface::Cursor.