Expand description
Index pickers for the count query.
Pure functions on the document type’s index map + where clauses;
no Drive, no proof. Picks a covering index for a given query
shape, returning None if no index can serve the query.
The clauses must already be keyed by
key_array_element_clauses,
which judges a clause on a typed array: a keyed element read and a raw
== or in on the array look alike here, and both select the array’s
index, whose entries are per element.