Adding index on a compound index on Mongodb

Question

I created a compound index on my db using:

db.collection.ensureIndex({a:1,b:1})

Now I realized I need another level in the composition:

db.collection.ensureIndex({a:1,b:1,c:1})

Will Mongodb create a whole new index, or will it know to modify the existing one?

JohnnyHK · Accepted Answer · 2015-05-15 20:08:03Z

5

Calling ensureIndex with different fields will create a new index.

You can confirm this after running both commands by using getIndexes to see what indexes exist for your collection:

db.collection.getIndexes()

If you no longer want the original index, you can drop it using:

db.collection.dropIndex({a:1,b:1})

answered May 15, 2015 at 19:57

JohnnyHK

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1 Comment

Just to add to this, the index {a:1,b:1,c:1} will behave just like {a:1,b:1} (in addition to supporting c) since mongo will use index prefixes.