Major - P3 It is confusing that explain() does not indicate whether an index filter was applied. Including this information can significantly help the slow-query debugging process.
> db.foo.insert({_id:0, a:1})
SingleWriteResult({
"writeErrors" : [ ],
"writeConcernErrors" : [ ],
"nInserted" : 1,
"nUpserted" : 0,
"nUpdated" : 0,
"nModified" : 0,
"nRemoved" : 0,
"upserted" : [ ]
})
> db.foo.runCommand("planCacheSetFilter",{query:{a:1},indexes:[{_id:1}]})
{ "ok" : 1 }
> db.foo.find({a:1}).hint({a:1}).explain()
{
"cursor" : "BasicCursor", // no indication as to why hinted index is not being used
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"server" : "Rassi-MacBook-Pro.local:27017"
}
>