MongoDB Index Best Practices

Sun, 10 Apr 2022 16:19:10 +0800

After the sorted operation OOM incident in production from the previous post, I realized my understanding of MongoDB indexes wasn’t as deep as it should be. I started searching extensively and finally organized some key points. Some were covered in the previous post — feel free to review MongoDB Sorted Operation OOM.

When to Use Indexes

When you have sorting scenarios and in-memory sorting exceeds 32 MB.
When a specific field has a uniqueness requirement.
When the document count is large.
Build indexes on high-cardinality fields.

When NOT to Use Indexes

When an index cannot effectively filter data.
Don’t set indexes on frequently updated fields.

How to Design and Use Indexes

Use Compound Indexes Instead of Single Field Indexes

In MongoDB, a find query can only use one index at a time (in most scenarios). So if your use case frequently filters on multiple fields, use a Compound Index composed of multiple fields to match your query conditions.

MongoDB Sorted Operation OOM

Sun, 20 Mar 2022 15:11:15 +0800

This is a rare case where I learned by working backward from a problem — I’d better document it well. Recently, our production environment hit a MongoDB sort operation limit with this specific error:

MongoDB.Driver.MongoCommandException: Command find failed: Encountered non-retryable error during query :: caused by :: Executor error during find command :: caused by :: Sort operation used more than the maximum 33554432 bytes of RAM.