[tracker/wip/carlosg/sqlite-3-35-workaround: 1/2] libtracker-data: Workaround SQLite 3.35.x bug

[Carlos Garnacho <carlosg src gnome org>]

commit a9aa8dfcc003e7b4cd3f9ebd94bb2a7499355564
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sun Mar 21 17:38:48 2021 +0100

    libtracker-data: Workaround SQLite 3.35.x bug
    
    The optimization for UNION ALLs inside JOINs had another unexpected
    victim:
    
      SELECT * { GRAPH ?g { ?a ... OPTIONAL { ?a ... } } }
    
    Does now break when matching the graph between both sides of the
    LEFT JOIN caused by OPTIONAL, making all other values coming from
    the right hand side come back empty. Make it sure this specific case
    is ineligible for query flattening optimizations by adding a LIMIT
    clause on the right hand side of the LEFT JOIN. This brings back
    correct over fast.
    
    This workaround should be revisited when a SQLite fix is available.

 src/libtracker-data/tracker-sparql.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index c4aaaaaa9..b2ed0e6b6 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -5085,8 +5085,28 @@ translate_OptionalGraphPattern (TrackerSparql  *sparql,
 
        _call_rule (sparql, NAMED_RULE_GroupGraphPattern, error);
 
-       if (do_join)
+       if (do_join) {
+               /* FIXME: This is a workaround for SQLite 3.35.x, where
+                * the optimization on UNION ALLs inside JOINs (Point 8c in
+                * the 3.35.0 release notes) break in this very specific
+                * case:
+                *
+                *   SELECT * { GRAPH ?g { ?a ... OPTIONAL { ?a ... } } }
+                *
+                * This is a workaround to make this one case ineligible
+                * for query flattening optimizations, specifically make
+                * it fall through case 8 in the list at
+                * https://sqlite.org/optoverview.html#flattening,
+                * "The subquery does not use LIMIT or the outer query is not
+                * a join.", we will now meet both here.
+                *
+                * This should be evaluated again in future SQLite version.
+                */
+               if (tracker_token_get_variable (&sparql->current_state->graph))
+                       _append_string (sparql, "LIMIT -1 ");
+
                _append_string (sparql, ") ");
+       }
 
        return TRUE;
 }