CODEWITHSUNDEEP
mongodb
Christian P
Christian P

Reputation: 12240

How does MongoDB evaluates multiple $or statements?

How will MongoDB evaluate this query:

db.testCol.find(
{
    "$or" : [ {a:1, b:12}, {b:9, c:15}, {c:10, d:"foo"} ]
});

When scanning values in a document if first OR statement is TRUE will the other statements be also be evaluated?

Logically if the MongoDB is optimized other values in OR statement should not be evaluated, but I don't know how MongoDB is implemented.

UPDATE: I updated my query because it was wrong and it didn't explain correctly what I was trying to accomplish. I need to find a set of documents that have different properties and if an exact combination of these properties is found the document must be returned.

The SQL equivalent of my query would be: 

SELECT * FROM testCol 
WHERE (a = 1 AND b = 12) OR (b = 9 AND c = 15) OR (c = 10 AND d = 'foo');

Upvotes: 3

Views: 1513

Answers (2)

Spencer
Spencer

Reputation: 727

EDIT: Mongo only skips documents during the de-duplication process, not during the table scans.

Mongo won't check documents that are already part of the result set. So if your first {a:1, b:12} returns 100% of the documents, Mongo is done.

You want to put whatever will grab the most documents as your first evaluated statement because of this. If your first item only grabs 1% of documents, the subsequent item will need to scan the other 99%.

That being said, you are using $or to look for values in a single key. I think you want to use $in for this.

See here for more:

http://books.google.com/books?id=BQS33CxGid4C&lpg=PA48&ots=PqvQJPRUoe&dq=mongo%20tips%20and%20tricks%20%22OR-query%22&pg=PA48#v=onepage&q&f=false

Upvotes: 1

Remon van Vliet
Remon van Vliet

Reputation: 18605

MongoDB will execute each clause of the $or operation as a seperate query and remove duplicates as a post processing pass. As such each clause can use a seperate index which is often very useful.

In other words, it will NOT look at 1 document, see which of the OR clauses apply and do an early-out if the first clause is a match. Rather it does a full dataset query per clause and de-dupe after the fact. This may seem less than efficient but in practice it's almost always faster since the first approach would only be able to hit at most one index for all clauses which is rarely efficient.

Upvotes: 3

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