How To Efficiently Design Nested OPTIONAL Clauses In SPARQL - sparql

Question

I want to retrieve data that has optional elements at multiple levels. For example, assume I have four ancestors - Fred, Sam, George, and Mark. Fred and Sam have kids ... George and Mark do not. All of Fred's kids have nicknames, but two of Sam's four kids do not.
I want to query all of the kids of my ancestors and return their names, ages, and nicknames.
It seems like this would work:
SELECT DISTINCT ?token ?ancestorName ?childName ?childAge ?childNickname
WHERE
{
FILTER ( ?token IN ("Fred","Sam","George","Mark") )
?ancestor foo:name ?token .
?ancestor foo:fullname ?ancestorName .
OPTIONAL
{
?ancestor foo:parentOf ?child .
?child foo:fullname ?childName .
?child foo:age ?childAge .
OPTIONAL { ?child foo:nickname ?childNickname }
}
}
Everything seems to work fine if an ancestor doesn't have a child ... all of the outer optional clause returns quickly
with no data. If the ancestor has children, and each has a nickname, it returns quickly and fills in the data. The problem seems to happen when the ancestor has a child, but the child does not have a nickname.
It works ... but it takes a very long time (I have lots of data). It appears that the inner OPTIONAL clause
OPTIONAL { ?child foo:nickname ?childNickname }
... does a cross product ... combining every ?child with every ?childNickname ... and then returns the right value.
How can I write this SPARQL SELECT to run efficiently (not do a cross product) and return all of the ancestors and all of the kids even if a kid doesn't have a nickname? I've tried FILTERS. I've tried checking whether ?child was BOUND. I haven't found the secret to make it run quick.
Thanks for the help!