For this OR query: <pre class="prettyprint"><code>db.messages.find({ $or: [ { to: { $ne: null }, from: "xyz" }, { to: "xyz" } ] }).sort({_id: -1}).limit(50) </code></pre> with following indices: <pre class="prettyprint"><code>{to:1, from: 1, _id:-1} and {from:1, to:1, _id:-1} </code></pre> mongo is always doing a full scan. I was hoping that mongo could use these two indices and merge the results. Do I need to split this into two queries (one for each OR clause) and merge myself? Or is there some other index that can help?

Compound indices for OR+Sort query in mongodb

For this OR query:

db.messages.find({ $or: [ { to: { $ne: null }, from: "xyz" }, { to: "xyz" } ] }).sort({_id: -1}).limit(50)

with following indices:

{to:1, from: 1, _id:-1} and {from:1, to:1, _id:-1}

mongo is always doing a full scan.

I was hoping that mongo could use these two indices and merge the results.

Do I need to split this into two queries (one for each OR clause) and merge myself? Or is there some other index that can help?

This is a known issue, https://jira.mongodb.org/browse/SERVER-1205, which you can vote for if it is very important for your use case.