sql: Add Query.__bool__

Makes it clearer what the intent is (rather than implicitly falling back
on __len__ via Python) and also happens to be a small (most probably
insignificant) performance improvement.

On my machine, without a __bool__:

  Name (time in us)            Min       Max    Median
  test_bool_benchmark     144.2940  879.5960  167.2730

With it:

  Name (time in us)            Min       Max    Median
  test_bool_benchmark     133.3990  876.1080  152.1879

2 files changed, 26 insertions, 0 deletions

diff --git a/qutebrowser/misc/sql.py b/qutebrowser/misc/sql.py
index 7844253f0..090c3a5d2 100644
--- a/qutebrowser/misc/sql.py
+++ b/qutebrowser/misc/sql.py
@@ -394,6 +394,12 @@ class SqlTable(QObject):
         q.run()
         return q.value()
 
+    def __bool__(self):
+        """Check whether there's any data in the table."""
+        q = Query(f"SELECT 1 FROM {self._name} LIMIT 1")
+        q.run()
+        return q.query.next()
+
     def delete(self, field, value, *, optional=False, like=False):
         """Remove all rows for which `field` equals `value`.
 
diff --git a/tests/unit/misc/test_sql.py b/tests/unit/misc/test_sql.py
index 3edbbffb0..ab34727a9 100644
--- a/tests/unit/misc/test_sql.py
+++ b/tests/unit/misc/test_sql.py
@@ -255,6 +255,26 @@ def test_len():
     assert len(table) == 3
 
 
+def test_bool():
+    table = sql.SqlTable('Foo', ['name'])
+    assert not table
+    table.insert({'name': 'one'})
+    assert table
+
+
+def test_bool_benchmark(benchmark):
+    table = sql.SqlTable('Foo', ['number'])
+
+    # Simulate a history table
+    table.create_index('NumberIndex', 'number')
+    table.insert_batch({'number': [str(i) for i in range(100_000)]})
+
+    def run():
+        assert table
+
+    benchmark(run)
+
+
 def test_contains():
     table = sql.SqlTable('Foo', ['name', 'val', 'lucky'])
     table.insert({'name': 'one', 'val': 1, 'lucky': False})