WiredTiger prefix compression for index on MongoDB

Question

According to the MongoDB documentation (at least this blog entry https://www.mongodb.com/blog/post/new-compression-options-mongodb-30), prefix compression is enabled by default on MongoDB.

However, when I look at one collection, I see this:

db.myCollection.stats()
{
...,
    "wiredTiger" : {
        "metadata" : {
            "formatVersion" : 1
        },
        "creationString" : "...,prefix_compression=0,prefix_compression_min=4"
...
}

Does prefix_compression=0 means that my indexes are not prefix-compressed ?

Answer 1

By default collection stats only include information on the collection data (which uses block-level compression rather than prefix compression).

To see index details you need to provide an additional indexDetails option, eg:

db.myCollection.stats({indexDetails:true}).indexDetails._id_.creationString

You should see "prefix_compression=true" in the index creationString value.