Question

Are there any performance impacts using javascript expressions inside mongodb query instead standard BSON notation. For example:

>db.myCollection.find( { a : { $gt: 3 } } );
>db.myCollection.find( { $where: "this.a > 3" } );

Will the first query be faster than the second one if there is no index on a column? Also, is there any way to write query

>db.myCollection.find( { $where: "this.a / 10 > 3" } );

or

>db.myCollection.find( { $where: "this.a / this.b > 3" } );

without using $where notation?

Answer 1

Will the first query be faster than the second one if there is no index on a column?

In both cases the first on will basically always be faster. The $where clause will use the javascript engine. There can only be one javascript engine running per instance, so it may have to wait. Additionally, the $where clause will have to move objects into and out of the javascript VM which adds overhead and will be slower.

Also, is there any way to write query… .find( { $where: "this.a / this.b > 3" } ); without using $where notation?

The answer here is no. The query engine does not support any query where comparing data within an object. The only workarounds here are to:

1. Pre-compute the column (which I’m sure you’re trying to avoid)
2. Run a for loop somewhere and do this “manually”. This can be done with a Map/Reduce or a client-side query.

Of course, both solutions are sub-optimal. This is definitely a whole in MongoDB’s functionality.