optimizing query for $exists in sub property

问题

I need to search for the existence of a property that is within another object. the collection contains documents that look like:

"properties": {
    "source": {
        "a/name": 12837,
        "a/different/name": 76129
    }
}

As you can see below, part of the query string is from a variable.

With some help from JohnnyHK (see mongo query - does property exist? for more info), I've got a query that works by doing the following:

var name = 'a/name';
var query = {};
query['properties.source.' + name] = {$exists: true};
collection.find(query).toArray(function...

Now I need to see if I can index the collection to improve the performance of this query.

I don't have a clue how to do this or if it is even possible to index for this.

Suggestions?

回答1:

2 things happening in here.

First probably you are looking for sparse indexes.

http://docs.mongodb.org/manual/core/index-sparse/

In your case it could be a sparse index on "properties.source.a/name" field. Making indexes on field will dramatically improve your query lookup time.

db.yourCollectionName.createIndex( { "properties.source.a/name": 1 }, { sparse: true } )

Second thing. Always when you want to know whether your query is fast/slow, use mongo console, run your query and on its result call explain method.

db.yourCollectionName.find(query).explain(); Thanks to it you will know whether your query uses indexes or not, how many documents it had to check in order to complete query and some others useful information.

来源：https://stackoverflow.com/questions/30312836/optimizing-query-for-exists-in-sub-property