diff --git a/src/calibre/db/search.py b/src/calibre/db/search.py
index bd8853cbe0..b4e1c62a21 100644
--- a/src/calibre/db/search.py
+++ b/src/calibre/db/search.py
@@ -671,7 +671,7 @@ class Parser(SearchQueryParser):  # {{{
             matches = set()
             error_string = '*@*TEMPLATE_ERROR*@*'
             template_cache = {}
-            global_vars = {}
+            global_vars = {'_candidates': candidates}
             for book_id in candidates:
                 mi = self.dbcache.get_proxy_metadata(book_id)
                 val = mi.formatter.safe_format(template, {}, error_string, mi,
diff --git a/src/calibre/gui2/dialogs/search.py b/src/calibre/gui2/dialogs/search.py
index 4204036a1b..f5794fc335 100644
--- a/src/calibre/gui2/dialogs/search.py
+++ b/src/calibre/gui2/dialogs/search.py
@@ -294,7 +294,15 @@ def create_template_tab(self):
     self.template_program_box = le = TemplateLineEditor(self.tab_widget)
     le.setObjectName('template_program_box')
     le.setPlaceholderText(_('The template that generates the value'))
-    le.setToolTip(_('Right click to open a template editor'))
+    le.setToolTip('<p>' +
+                  _('Right click to open a template editor. <br>'
+                    'Technical note: the set of book ids already matched by '
+                    'previous search terms in the search expression is passed '
+                    'to the template in the global variables dictionary with the '
+                    'key "{0}". You can use the set to limit any work the '
+                    'template does to the set of books already matched, possibly '
+                    'improving performance.'
+                    ).format('_candidates')  + '</p>')
     lo = QHBoxLayout()
     lo.addWidget(le)
     self.edit_template_button = tb = QToolButton()