[tracker/wip/carlosg/sparql1.1: 167/201] libtracker-data: Work around terrible query planning from SQLite

[Carlos Garnacho <carlosg src gnome org>]

commit 71170665dda8614254728ac99471f1e43e566108
Author: Carlos Garnacho <carlosg gnome org>
Date:   Mon Jul 15 22:02:16 2019 +0200

    libtracker-data: Work around terrible query planning from SQLite
    
    Using a multivalued field as a property function on the union graph
    incurs in very expensive query planning, as full table scans happen
    for all selects in the union view.
    
    This makes queries like "select rdf:type(?u) { ?u a rdfs:Resource }"
    pretty expensive pretty quickly. And that's precisely what grilo asks
    anyone using the Tracker plugin to do.
    
    Work around this situation by doing the query in a way that indexes
    are used, and we get non-Ent speeds back.

 src/libtracker-data/tracker-sparql.c | 59 ++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index cbda7147a..af87105a1 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -6488,6 +6488,65 @@ handle_property_function (TrackerSparql    *sparql,
 {
        TrackerPropertyType type;
 
+       /* As of SQLite 3.26.0, performing an aggregate function (or anything
+        * that requires scanning all results, like distinct) on a unionGraph
+        * view results in full table scans on all unioned selects.
+        *
+        * This quickly gets far too expensive on property functions, as they
+        * are normally used in the SelectClause, so they get to run once per
+        * result. This makes queries like:
+        *
+        * SELECT rdf:type(?u) { ?u a rdfs:Resource }
+        *
+        * prohibitive. The solution is to perform the union inline, and
+        * evaluate the ArgList once on each of the unioned selects. This again
+        * hits table search, and hopefully through an index, so it gets fast
+        * again.
+        */
+       if (tracker_property_get_multiple_values (property) &&
+           tracker_token_is_empty (&sparql->current_state.graph)) {
+               TrackerStringBuilder *str, *old;
+               TrackerParserNode *arg;
+               GHashTable *ht;
+               GHashTableIter iter;
+               gpointer graph;
+
+               arg = _skip_rule (sparql, NAMED_RULE_ArgList);
+
+               _append_string (sparql, "(SELECT GROUP_CONCAT (");
+               str = _append_placeholder (sparql);
+               old = tracker_sparql_swap_builder (sparql, str);
+               _append_string_printf (sparql, "\"%s\"", tracker_property_get_name (property));
+               convert_expression_to_string (sparql, tracker_property_get_data_type (property));
+               tracker_sparql_swap_builder (sparql, old);
+
+               _append_string (sparql, ", ',') ");
+
+               _append_string_printf (sparql, "FROM (SELECT \"%s\" FROM \"main\".\"%s\" WHERE ID = ",
+                                      tracker_property_get_name (property),
+                                      tracker_property_get_table_name (property));
+               if (!_postprocess_rule (sparql, arg, NULL, error))
+                       return FALSE;
+
+               ht = tracker_data_manager_get_graphs (sparql->data_manager);
+               g_hash_table_iter_init (&iter, ht);
+
+               while (g_hash_table_iter_next (&iter, (gpointer *) &graph, NULL)) {
+                       _append_string_printf (sparql, "UNION ALL SELECT \"%s\" FROM \"%s\".\"%s\" WHERE ID = 
",
+                                              tracker_property_get_name (property),
+                                              (gchar *) graph,
+                                              tracker_property_get_table_name (property));
+
+                       if (!_postprocess_rule (sparql, arg, NULL, error))
+                               return FALSE;
+               }
+
+               _append_string (sparql, ")) ");
+
+               sparql->current_state.expression_type = TRACKER_PROPERTY_TYPE_STRING;
+               return TRUE;
+       }
+
        if (tracker_property_get_multiple_values (property)) {
                TrackerStringBuilder *str, *old;