Describe how to include popularity information in the search index.
If there's no query or sort, sort by popularity

Without a query, there's no relevance score, so unless an explicit order
is given the sort order is essentially undefined.  Use the popularity
field in this case to return the most popular items first, because this
is probably useful and almost free to do.
Separate index names into content and auxiliary.

The page-traffic index is an example of an auxiliary index; it doesn't
hold content, but is used outside the main query (in this case, at
indexing time). We'll shortly be adding another such index to hold
best-bets.

Sitemaps should be made only from content indexes.  The search server
needs to know all the allowed names for its validation of index names
when opening indexes.