CODEWITHSUNDEEP
mongodb
bitinn
bitinn

Reputation: 9338

How does large array cause performance issue in MongoDB

Say we would like to store keywords as an array in mongodb and index them for faster look up, how does common performance issue with large array indexes apply?

{
  text: "some text",
  keyword: [ "some", "text" ]
}

Depending on the length of text, the keyword set might get quite large. If we set index as background, does it mitigate the slow down during document insertion? We are unlikely going to modify a keyword once it's created.

PS: we know about the experimental text search in mongodb, but some of our texts are not in the list of supported languages (think CJK), so we are considering a simple home-brew solution.

Upvotes: 0

Views: 3668

Answers (1)

Derick
Derick

Reputation: 36774

The issue that is mentioned in the "common performance issue" link that you point talks about modifying the array later. If you keep pushing elements to an array, MongoDB will need to move the document on disk. When it moves a document on disk, all the indexes that point to the moved document also need to be updated.

In your case, you will not be modifying the arrays, so there is no performance degradation due to moving documents around.

I don't think you even need to turn on background indexes. This is a feature that is meant for relieving locking on the database when you add an index to an already existing collection. Depending on the collection, the index build can take a long time and hence you would benefit from sacrificing some index building time for not-blocking your collection. If the index already exist, then the index-update time is so low that the time to add the document to the index is negligible compared to actually adding the document.

Upvotes: 3

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