MongoDb Fast Queries Javascript

Question

I am reading that document: http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24mod

$mod

The $mod operator allows you to do fast modulo queries to replace a common case for where clauses. For example, the following $where query:

db.things.find( "this.a % 10 == 1")

can be replaced by:

db.things.find( { a : { $mod : [ 10 , 1 ] } } )

So I didn't understand what fast means here. Performance?

aurora · Accepted Answer · 2011-10-14 10:07:23Z

3

I have not benchmarked this, but it will probably indeed mean performance. Apparently the "$where" executes javascript for each object, but "$mod" is a mongodb native operator, which should be much faster, because there is no need to execute any javascript for each object. Have also a look at the following sentence from the documentation:

Javascript executes more slowly than the native operators listed on this page, 
but is very flexible.

http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-JavascriptExpressionsand%7B%7B%24where%7D%7D

answered Oct 14, 2011 at 10:07

aurora

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3 Comments

kamaci Over a year ago

Do you have any idea about @Andrew Orsich's index answer?

aurora Over a year ago

For javascript queries it's not possible to use indexes, because the javascript has to be evaluated for each object, which is selected with your query. So: if you have the choice between a built-in operator like $mod and a javascript, the javascript should always be slower, because the javascript is evaluated for each object and it can not use an index, while the builtin operator is evaluated only ones when your query is parsed by mongodb and it can use an index. It would probably be interesting, to benchmark this ...

kamaci Over a year ago

Thanks for your answer. Just one more. What you mean with indexing can I read it from anywhere? I think I don't know what.

Andrew Orsich · Accepted Answer · 2011-10-14 12:06:40Z

1

Any javascript/regex queries in mongodb can't use indexes and work slow. So answer on your question is yes, documentation say about performance.

More info about server side javascript you can find here

answered Oct 14, 2011 at 12:06

Andrew Orsich

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