Re: [Tracker] tracker-sparql -q QUERY shows results while exact same query via SPARQL C API doesn't

[Sam Thursfield <ssssam gmail com>]

Hi Frank

On Mon, Oct 22, 2012 at 10:38 AM, Frank Lahm <franklahm gmail com> wrote:
> Hi,
>
> is there anything wrong with this SPARQL query
>
>   "SELECT ?url WHERE { ?f nie:url ?url . ?f nfo:fileName ?name .
> FILTER(fn:starts-with(?url, 'file:///Volumes/test/') && regex(?name,
> 'unti')) }"
>
> which would explain why tracker-sparql -q QUERY shows results:
>
> frank$ tracker-sparql -q "SELECT ?url WHERE { ?f nie:url ?url . ?f
> nfo:fileName ?name . FILTER(fn:starts-with(?url,
> 'file:///Volumes/test/') && regex(?name, 'unti')) }"
> Ergebnisse:
>   file:///Volumes/test/untitled%20text
>
> ...but the exact same query from a program [1] that uses the SPARQL C
> API would give no result ?
>
> I'm 100% sure it is the exact same query as I have a debug log
> statement logging the query string it just a few lines before calling
> tracker_sparql_connection_query():

One non-obvious thing that can change how queries are run is the
LOCALE / LC_COLLATE setting. If you have non-Western characters in
your results and your own C program is running in a different locale
to tracker-sparql for some reason, it might explain the issue. It
doesn't seem likely in this case though.

>   Oct 17 22:03:41.777237 afpd[27868] {spotlight_module.c:153}
> (D5:Spotlight): sl_mod_start_search: SPARQL query:
>   "SELECT ?url WHERE { ?f nie:url ?url . ?f nfo:fileName ?name .
> FILTER(fn:starts-with(?url, 'file:///Volumes/test/') && regex(?name,
> 'test')) }"
>
> Other SPARQL queries like
>
>   "SELECT ?url WHERE { ?f nie:url ?url FILTER regex(?url, 'test')}"
>
> work just fine from the program (and tracker-sparql). It seems as as
> soon as I have FILTER expressions with more then one sub-expressions
> (or more then one FILTER expression) I get this behaviour.
>
> Unfortunately I can't seem to find a way to get Tracker to log debug
> messages for the queries from the C program, the tracker-control
> --set-log-verbosity=debug doesn't show me any query info at all (in
> /root/.local/share/tracker/tracker-store.log).
>
> I'm puzzled and would appreciate any pointers. Thanks!


By default libtracker-sparql uses direct access for read queries,
meaning that it loads the Tracker database in your application process
and executes the query there - therefore, the tracker-store logs won't
show any evidence of the access because the query didn't reach the
tracker-store process itself at all. Write queries always go over DBus
to the tracker-store process, so you should see logging information of
those.

I believe if you set TRACKER_VERBOSITY=3 in the environment of your C
program you'll see the logs of the queries that are being run
in-process.

I hope some of this helps
Sam