Optimize SPARQL query to load SAMPLE labels

Question

The following query loads contracts from my data set (a contract is between an organization and a partner).

SELECT ?contract ?organisation ?partner
WHERE {
    ?organisation
        a gr:BusinessEntity ;
        rejstriky:contract ?contract .

    ?contract a rejstriky:Contract ;
        rejstriky:partner ?partner .
}
GROUP BY ?contract ?organisation ?partner

This query returns around 8000 contracts and it does that immediately (it takes just a fraction of second). Now I need to load labels/names for both the organization and the partner. There might be multiple names available, I just need one. This is my query:

SELECT ?contract ?organisation ?partner
    (SAMPLE(?organisationNames) AS ?organisationName)
    (SAMPLE(?partnerNames) AS ?partnerName)
WHERE {
    ?organisation
        a gr:BusinessEntity ;
        rejstriky:contract ?contract .

    ?contract a rejstriky:Contract ;
        rejstriky:partner ?partner .

    ?organisation gr:legalName ?organisationNames .
    ?partner gr:legalName ?partnerNames .
}
GROUP BY ?contract ?organisation ?partner

This query suddenly takes several minutes to finish .

I did some experiments and I found out that if I decided to get all the names using separate SPARQL calls (by 40 names in a single batch), it'd take less than 2 minutes (it would be significantly faster). Regardless of that, if I'm able to generate those 8000 items within a fraction of second, loading two labels for each item should not take that long.

Do you have any ideas how to optimize my query? Note that I'm using Virtuoso.

Answer 1

Not having access to sample data or Virtuoso it's difficult to be sure if this will help, but you might try avoiding the use of SAMPLE.

 SELECT ?contract ?organisation ?organisationName ?partner ?partnerName

WHERE {
    ?organisation
        a gr:BusinessEntity ;
        rejstriky:contract ?contract .

    ?contract a rejstriky:Contract ;
        rejstriky:partner ?partner .

   { SELECT ?organisationName WHERE { ?organisation gr:legalName ?organisationName . } LIMIT 1}
   { SELECT ?partnerName WEHRE {?partner gr:legalName ?partnerName . } LIMIT 1}
}
GROUP BY ?contract ?organisation ?organisationName ?partner ?partnerName