Facets and docValues

[Chris Ulicny]

I've been trying to figure out how exactly docValues help with facet
queries, and I only seem to find mention that they are beneficial to facet
performance without many specifics. What I'd like to know is whether it
applies to all fields used in the facet or just fields that are faceted on.

For example, consider if we have the following facet

catfacet:{
    type: terms,
    field: category,
    facet: {
        x_sum:"sum(x)",
        y_sum:"sum(y)"
    }
}

Is it beneficial to have docValues enabled for all three fields used or
some specific subset of them?

Thanks.