Yes, they are pointers. But the indexed fields have to be duplicated for efficient reading. From Wikipedia:

"An index is a copy of select columns of data from a table that can be searched very efficiently that also includes a low-level disk block address or direct link to the complete row of data it was copied from." [1]

When you reference a field in an index, you're effectively duplicating this column on disk.

We can discuss semantics, but at the end of the day, indexes are glorified copies of your columns in a specific order.

[1] https://en.wikipedia.org/wiki/Database_index

Not only that, they're projections which you don't have to spend effort on keeping up to date, and which the engine automatically uses to optimize your queries. While there's an argument to be made that the same can be accomplished in a non-relational database with suitable application code or a translation layer, all else equal it's better not to need those things in the first place.

And as we saw from a recent Uber post about Postgresql, that automatic updating is not actually free.

Of course it's not, but if you're not Uber, that almost certainly isn't a problem for you. When you use MongoDB, you get to roll your own indexes no matter who you are.

Was going to say the same, but I realized the OP was talking about in your code, not so much as overhead.