MongoDB fulltext search not using index

问题

We use mongoDB fulltext search to find products in our database. Unfortunately it is incredible slow. The collection contains 89.114.052 documents and I have the suspicion, that the full text index is not used. Performing a search with explain(), nscannedObjects returns 133212. Shouldn't this be 0 if an index is used?

My index:

{
    "v" : 1,
    "key" : {
        "_fts" : "text",
        "_ftsx" : 1
    },
    "name" : "textIndex",
    "ns" : "search.products",
    "weights" : {
        "brand" : 1,
        "desc" : 1,
        "ean" : 1,
        "name" : 3,
        "shop_product_number" : 1
    },
    "default_language" : "german",
    "background" : false,
    "language_override" : "language",
    "textIndexVersion" : 2
}

The complete test search:

> db.products.find({ $text: { $search: "playstation" } }).limit(100).explain()
{
    "cursor" : "TextCursor",
    "n" : 100,
    "nscannedObjects" : 133212,
    "nscanned" : 133212,
    "nscannedObjectsAllPlans" : 133212,
    "nscannedAllPlans" : 133212,
    "scanAndOrder" : false,
    "nYields" : 1041,
    "nChunkSkips" : 0,
    "millis" : 105,
    "server" : "search2:27017",
    "filterSet" : false
}

回答1:

Please have a look at the question you asked:

".... The collection contains 89.114.052 documents and I have the suspicion, that the full text index is not used ...."

You are only "nScanned" for 133212 documents. Of course the index is used. If it was not then 89,114,052 documents ( because this is English locale and not German ) would have otherwise been reported in "nScanned" which means an index is not used.

Your query is slow. Well it seems your hardware is not up to the task of keeping 1333212 documents in memory or otherwise having the super fast disk to "page" effectively. But this is not a MongoDB problem but yours.

You have over 100,000 documents that match your query and even if you just want 100 then you need to accept this is how this works and MongoDB does not "give up" once you have matched 100 documents and yield control. The query pattern here finds all of the matches and then applies the "limit" to the cursor in order just to return the most recent.

Maybe some time in the future the "text" functionality might allow you do do things like you can do in the aggregate version of $geoNear and specify "minimum" and "maximum" values for a "score" in order to improve results. But right now it does not.

So either upgrade your hardware or use an external text search solution if your problem is the slow results on matching over 100,000 documents out of over 89,000,000 documents.

来源：https://stackoverflow.com/questions/24825551/mongodb-fulltext-search-not-using-index