Graph statistics give the number of for "S P O" where S, P or O can be a wildcard.
This didn't quite work out for query optimization - the needs for optimization are a little more complicated, needing information such as "if S is going to fixed, estimate the number of "S P O" with out yet knowing what S is fixed as, or it might be one of several values.
The only significant implementation is GraphMemStatisticsHandler for the plain memory graphs. It does exploit the mem indexes to get the count, compared to counting a "find" but the improvement appears to be small.
Proposal: deprecate GraphStatisticsHandler, retain the mem code "just in case".
This will clear the way for either removal or incompatible change in the future.