sparql slow if more object value specified

[anatta8]

Hi , I am using Virtuoso 7 open source edition,

This query below is fast, it returns the result instantly,

select *
                WHERE {

                graph <http://localhost:8890/graph> {

                    ?catAtt qq:catId        ?catId;
                            qq:caDataType   ?caDataType;
                            qq:showInView   ?showInview;
                            qq:valFormat    ?valFormatKey;
                            qq:multiple     ?multiple; 
                            qq:position     ?position; 
                            qq:link         ?link; 
                            qq:catAttName   ?catAttName; 
                            qq:setting      ?setting; 
                            qq:flag         ?flag;
                            qq:unit         ?caUnit.

                  }  
                }

                LIMIT 20

This query is so slow, took 10 seconds for the result, the only difference is the ?catId is replaced with literal value 1. I thought this should be faster because it's more specific.

select *
                WHERE {

                graph <http://localhost:8890/graph> {

                    ?catAtt qq:catId        1;
                            qq:caDataType   ?caDataType;
                            qq:showInView   ?showInview;
                            qq:valFormat    ?valFormatKey;
                            qq:multiple     ?multiple; 
                            qq:position     ?position; 
                            qq:link         ?link; 
                            qq:catAttName   ?catAttName; 
                            qq:setting      ?setting; 
                            qq:flag         ?flag;
                            qq:unit         ?caUnit.

                  }  
                }

                LIMIT 20

Why is it so? I am still new to SPARQL and triplestore, seems like I have to do some indexing on the object?

I have tried creating the GOPS index with, I don't understand about the Partition part, is using O correct? What does that partition mean? Creating this index has improved the execution for the second query to 4 seconds, but still too slow for a DBMS.

CREATE COLUMN INDEX RDF_QUAD_GOPS
  ON RDF_QUAD (G, O, P, S)
  PARTITION (O VARCHAR (-1, 0hexffff));

Btw, does virtuoso stored triplet in the regular RDBMS table? in the table DB.DBA.RDF_QUAD? So Virtuoso does not have a native storage of graph? Or what I don't understand?

Thanks

[HughWilliams]

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