Question

I have a index {s:1} on collection k like:

Data in k likes :


{ s:"aaa"} ,
{s:"bbb"} ,
{s:""}


Is it safe to use {s:{$gt:""}} to find out every item that the length of s greater than 0 ?

Answer 1

I think it’s “safe” based on the fact:

• Any non-empty string is greater than empty string in lexicographical order.
• The index works properly so performance should be ok.

To test if index works properly, you can execute

db.collectionName.find({s: {$gt: ""}}).explain()

When seeing "cursor" : "BtreeCursor s_1", it means the index worked.

However, do you consider to modify your application logic and remove s field when it’s empty?

• db.test.find({s: {$exists: false}}).hint({s: 1}) can be used in this query. It is more nature than $gt.
• You can benefit from MongoDB sparse index in performance. Especially when most of your data are with empty s.