Combine SELECT and ASK queries in a same query - sparql

Question

Context: 14M triples, Blazegraph workbench. I'm currently attempting to design queries which combine SELECT and ASK. More exactly, I want to select results in my graph where an assumption is true.
For my example, imagine I've many books which have one author and one editor. I want to select the book from the author which his book is linked through random path length property to the client#1.
In my case, with my data, it takes a lot of time to realise the query directly like that:
SELECT ?id_book
WHERE {?id_book prefix:hasAuthor :author#1.
?id_book prefix:linkedToEditor*/prefix:hasClient :client#1}
ORDER by ?id_book
To reduce the time of calculus (x 1:1000), I'm using a script to realise these queries successively. The script selects the books which have as author the author n°1:
SELECT ?id_book
WHERE {?id_book prefix:hasAuthor :author#1}
ORDER by ?id_book
And I ask for each result for 1 to n (id_book#1, id_book#2, ..., id_book#n) if it's linked to client n°1:
ASK {id_book#i prefix:linkedToEditor*/prefix:hasClient :client#1}
The SELECT query followed by the ASK query is far faster than the first SELECT query for the same results. I don't want to explore all the possibilities of ?id_book prefix:linkedToEditor*/prefix:hasClient :client#1; I just want to save results where the link exists. I tried with FILTER EXISTS or two SELECT queries, but the query times are similarly long:
SELECT ?id_book
WHERE {?id_book prefix:hasAuthor :author#1.}
FILTER EXIST {?id_book prefix:linkedToEditor*/prefix:hasClient :client#1}
ORDER by ?id_book
or
SELECT ?id_book
WHERE {?id_book prefix:linkedToEditor*/prefix:hasClient :client#1.
{SELECT ?id_book
WHERE {?id_book prefix:hasAuthor :author#1.}
}
}
How can I optimise my queries into one query?

Answer 1

It's a bit surprising that there's such a difference in your query times; a SPARQL engine should probably be able optimize the query to perform the simple part first, and then do the more complicated query property path afterward. The ordering could also cause some time increase, and it's really not important if you're just interested in boolean results.
At any rate, since nested queries are performed innermost first, you can sort of force the "do this first, then do that" by nesting the queries like this:
select ?id_book {
#-- first, get the books by author one
{ select ?id_book { ?id_book prefix:hasAuthor :author#1 } }
#-- then, then check that the book is related to client one
?id_book prefix:linkedToEditor*/prefix:hasClient :client#1
}
order by ?id_book