Small performance optimization when fetching items from library.

Fetch flexible attributes once for all items instead of once per
item. For queries with larger result sets this drastically cuts
down the number of issued sqlite queries.

beets/dbcore/db.py

@@ -565,6 +565,13 @@ class Results(object):
         a `Results` object a second time should be much faster than the
         first.
         """
+
+        # First fetch all flexible attributes for all items in the result.
+        # Doing the per-item filtering in python is faster than issuing
+        # one query per item to sqlite.
+        item_ids = [row['id'] for row in self._rows]
+        flex_attrs = self._get_flex_attrs(item_ids)
+
         index = 0  # Position in the materialized objects.
         while index < len(self._objects) or self._rows:
             # Are there previously-materialized objects to produce?
@@ -577,7 +584,10 @@ class Results(object):
             else:
                 while self._rows:
                     row = self._rows.pop(0)
-                    obj = self._make_model(row)
+                    if flex_attrs:
+                        obj = self._make_model(row, flex_attrs[row['id']])
+                    else:
+                        obj = self._make_model(row)
                     # If there is a slow-query predicate, ensurer that the
                     # object passes it.
                     if not self.query or self.query.match(obj):
@@ -599,20 +609,31 @@ class Results(object):
             # Objects are pre-sorted (i.e., by the database).
             return self._get_objects()
 
-    def _make_model(self, row):
-        # Get the flexible attributes for the object.
+    def _get_flex_attrs(self, ids):
+        # Get the flexible attributes for all ids.
         with self.db.transaction() as tx:
+            id_list = ', '.join(str(id) for id in ids)
             flex_rows = tx.query(
-                'SELECT * FROM {0} WHERE entity_id=?'.format(
-                    self.model_class._flex_table
-                ),
-                (row['id'],)
+                'SELECT * FROM {0} WHERE entity_id IN ({1})'.format(
+                    self.model_class._flex_table,
+                    id_list
+                )
             )
 
+        # Index flexible attributes by the entity id they belong to
+        flex_values = dict()
+        for row in flex_rows:
+            if row['entity_id'] not in flex_values:
+                flex_values[row['entity_id']] = dict()
+
+            flex_values[row['entity_id']][row['key']] = row['value']
+
+        return flex_values
+
+    def _make_model(self, row, flex_values={}):
         cols = dict(row)
         values = dict((k, v) for (k, v) in cols.items()
                       if not k[:4] == 'flex')
-        flex_values = dict((row['key'], row['value']) for row in flex_rows)
 
         # Construct the Python object
         obj = self.model_class._awaken(self.db, values, flex_values)