[GitHub] grahamtriggs commented on issue #527: SDB Transform - improve performance of OPTIONAL, MINUS

[GitBox <gi...@apache.org>]

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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