Dedup DNS attribution rows in one pass, not per candidate row

app/src/main/java/eu/faircode/netguard/DatabaseHelper.java

@@ -1166,25 +1166,28 @@ public Cursor getQAName(int uid, String ip, boolean alive) {
             SQLiteDatabase db = readableDb;
             String escapedIp = ip.replace("'", "''");
             String aliveFilter = alive
-                    ? " AND (%1$s.time IS NULL OR %1$s.time + %1$s.ttl >= " + now + ")"
+                    ? " AND (d.time IS NULL OR d.time + d.ttl >= " + now + ")"
                     : "";
             // There is a segmented index on resource. A shared IP can carry
             // DNS evidence for several qnames; keep only the most recently
             // observed row per qname (dedup) and order qnames by recency, so
             // the freshest resolution — most likely tied to the connection
             // that's actually being made now — is attributed first instead
             // of an alphabetically-first but possibly stale one.
-            String query = "SELECT d.qname, d.aname, d.time, d.ttl" +
+            //
+            // The dedup deliberately uses a single MAX(time) aggregate: with
+            // exactly one min/max aggregate, SQLite takes the bare columns
+            // from the row that supplied the maximum, so this is one index
+            // range scan over the IP's rows. A correlated per-row subquery
+            // here re-scans the IP's rows once per candidate row — O(n²) —
+            // and this query runs for every new connection (log() and
+            // blockKnownTracker()), where it grows with DNS history until
+            // it shows up as battery drain and heat.
+            String query = "SELECT d.qname, d.aname, d.time, d.ttl, MAX(d.time)" +
                     " FROM dns AS d" +
                     " WHERE d.resource = '" + escapedIp + "'" +
-                    String.format(aliveFilter, "d") +
-                    " AND d.ID = (" +
-                    "   SELECT d2.ID FROM dns AS d2" +
-                    "   WHERE d2.resource = d.resource AND d2.qname = d.qname" +
-                    String.format(aliveFilter, "d2") +
-                    "   ORDER BY d2.time DESC, d2.ID DESC" +
-                    "   LIMIT 1" +
-                    " )" +
+                    aliveFilter +
+                    " GROUP BY d.qname" +
                     " ORDER BY d.time DESC, d.ID DESC";
             return db.rawQuery(query, new String[] {});
         } finally {

app/src/test/java/eu/faircode/netguard/DatabaseHelperDnsAttributionTest.java

@@ -59,6 +59,33 @@ public void repeatedObservationsOfSameQnameCollapseToFreshestRow() {
         }
     }
 
+    @Test
+    public void aliveFilterAppliesBeforeDedup() {
+        DatabaseHelper dh = DatabaseHelper.getInstance(RuntimeEnvironment.getApplication());
+        dh.clearDns();
+
+        String ip = "203.0.113.30";
+        // The freshest observation has already expired; an older one is still
+        // alive. With alive=true the expired row must not shadow the alive one.
+        // Rows are inserted directly because insertDns() clamps the TTL to the
+        // "ttl" preference floor, which would keep the fresh row alive.
+        long now = System.currentTimeMillis();
+        dh.getWritableDatabase().execSQL(
+                "INSERT INTO dns (time, qname, aname, resource, ttl) VALUES ("
+                        + (now - 10_000) + ", 't2.example.com', 'alive-cname.example.com', '"
+                        + ip + "', 3600000)");
+        dh.getWritableDatabase().execSQL(
+                "INSERT INTO dns (time, qname, aname, resource, ttl) VALUES ("
+                        + (now - 5_000) + ", 't2.example.com', 'expired-cname.example.com', '"
+                        + ip + "', 1000)");
+
+        try (Cursor c = dh.getQAName(-1, ip, true)) {
+            assertEquals(1, c.getCount());
+            assertTrue(c.moveToFirst());
+            assertEquals("alive-cname.example.com", c.getString(c.getColumnIndexOrThrow("aname")));
+        }
+    }
+
     private static ResourceRecord rr(long time, String qname, String aname, String resource, int ttl) {
         ResourceRecord rr = new ResourceRecord();
         rr.Time = time;