grahamtriggs commented on issue #527: SDB Transform - improve performance of OPTIONAL, MINUS URL: https://github.com/apache/jena/pull/527#issuecomment-462271798 @afs Hi - I'm not entirely sure what you want as an example. To see what it is doing, this is the simplest form of query: ``` SELECT ?sublect ?label WHERE { ?subject a foaf:Person. OPTIONAL { ?subject rdfs:label ?label . } } ``` which is transformed into the equivalent of ``` SELECT ?sublect ?label WHERE { ?subject a foaf:Person. OPTIONAL { ?subject a foaf:Person. ?subject rdfs:label ?label . } } ``` If you had a triple store that had a million rdfs:label assertions, but only 50 of those applied to resources declared as being of type foaf:Person, there would be a substantial difference between the performance of these queries - certainly when using MySQL, where the second query would do an index join between the type / label, which is then applied to the outer join, rather than creating a temporary table of 1 million labels in the first query.
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