[tracker/wip/carlosg/joining-services: 3/5] libtracker-sparql: Join services table queries on the right resource keys

[Carlos Garnacho <carlosg src gnome org>]

commit ee8e257ca16c93e88a800f58df1967a887b9990e
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed May 13 01:28:05 2020 +0200

    libtracker-sparql: Join services table queries on the right resource keys
    
    If we have to join a services table query with previous triple blocks, we
    want to observe the resource ID vs URN string. Avoid a natural join, and
    specify the variables to match ourselves so we have this control.
    
    This makes this query work:
    
    SELECT ?u {
      ?u tracker:indexed true .
      SERVICE <dbus:org.freedesktop.Tracker3.Miner.Files> {
        ?u a rdfs:Resource
      }
    }

 src/libtracker-data/tracker-sparql.c | 39 +++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index 7412edc13..601ad3b71 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -4616,6 +4616,7 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
        gchar *pattern_str, *escaped_str, *var_str;
        TrackerContext *context;
        GList *variables = NULL;
+       GList *join_vars = NULL, *l;
        TrackerToken service;
        GString *service_sparql;
        gboolean silent = FALSE, do_join;
@@ -4627,7 +4628,7 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 
        if (do_join) {
                _prepend_string (sparql, "SELECT * FROM (");
-               _append_string (sparql, ") NATURAL INNER JOIN (");
+               _append_string (sparql, ") AS Left INNER JOIN (");
        }
 
        context = tracker_triple_context_new ();
@@ -4668,6 +4669,13 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 
                var_str = _extract_node_string (node, sparql);
                var = _extract_node_variable (node, sparql);
+
+               /* Variable was used before in the graph pattern, preserve
+               * for later so we join on it properly.
+               */
+               if (do_join && tracker_variable_get_sample_binding (var))
+                       join_vars = g_list_prepend (join_vars, var);
+
                variables = g_list_prepend (variables, var);
                binding = tracker_variable_binding_new (var, NULL, NULL);
                tracker_binding_set_data_type (binding, TRACKER_PROPERTY_TYPE_STRING);
@@ -4696,8 +4704,33 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
        tracker_sparql_pop_context (sparql, TRUE);
        g_string_free (service_sparql, TRUE);
 
-       if (do_join)
-               _append_string (sparql, ") ");
+       if (do_join) {
+               _append_string (sparql, ") AS Right ");
+
+               for (l = join_vars; l; l = l->next) {
+                       TrackerBinding *sample;
+
+                       if (l == join_vars)
+                               _append_string (sparql, "ON ");
+                       else
+                               _append_string (sparql, "AND ");
+
+                       sample = TRACKER_BINDING (tracker_variable_get_sample_binding (l->data));
+
+                       if (sample && sample->data_type == TRACKER_PROPERTY_TYPE_RESOURCE) {
+                               _append_string_printf (sparql, "(SELECT Uri FROM Resource WHERE ID = Left.%s) 
",
+                                                      tracker_variable_get_sql_expression (l->data));
+                       } else {
+                               _append_string_printf (sparql, "Left.%s ",
+                                                      tracker_variable_get_sql_expression (l->data));
+                       }
+
+                       _append_string_printf (sparql, "= Right.%s ",
+                                              tracker_variable_get_sql_expression (l->data));
+               }
+       }
+
+       g_list_free (join_vars);
 
        return TRUE;
 }